Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

motors

and to

ensure complete circuit de-energization

for

safety.

Molded-case

circuit breaker operating

mechanisms wear

if

operated

too

often.

They should

not

be

used

in

lieu

of

motor contactors. Refer

to

ANSI/IEEE

141-1993.

Molded-case circuit breakers

are

available

in

sev-

eral types.

The

thermal-magnetic type

is the one

most

commonly

used. This type

has two

automatic trip

mechanisms, both

of

which operate

to

release

the

trip

bar.

The

thermal

or

bimetal trip element operates

in

response

to

overload currents

in an

inverse time man-

ner:

the

higher

the

current,

the

shorter

is the

response

time.

This element

is

designed

to

follow

the

heating

characteristics

of the

conductor,

and it

allows

for

tem-

porary overloads, such

as for

motor starting. Refer

to

Figure

8-14a.

The

magnetic trip element operates

instantaneously

(within

a

fraction

of a

cycle) upon

occurrence

of

high-current short circuits,

and the

trip

setting

is

usually eight

to ten

times

the

breaker-rated

current.

Many breakers have

a

magnetic trip

adjust-

ment

of five to ten

times

the

breaker rated current,

and

they

should

be

adjusted

to be as low as

practicable

for

maximum

short circuit protection (see Figure

8-14b).

Some circuit breakers have only magnetic trips.

In

pumping

stations, most

are

used

in

combination motor

starters where

the

overload

function

is

provided

in the

overload relay. These circuit breakers

are

often

known

as

motor circuit protectors (MCPs). MCPs have

adjustable

trips that should

be set

just above

the

motor

inrush current. Refer

to

manufacturer's literature

for

available sizes

and

adjustment ranges.

Fused circuit breakers

are

hybrid circuit breakers

with

special

fuses

that open

on

very high

fault

cur-

rents that would destroy

the

breaker. Fuse operation

releases

the

trip

bar and

opens

the

breaker

to

prevent

single-phasing.

Molded-case switches

are

circuit breakers without

trip

elements

and are

used

as

manual disconnects.

Some have nonadjustable magnetic trip units

for

self-

protection. Molded-case switches must

be

applied with

caution, because

fault

currents near their interrupting

rating

may

force

the

contacts

to

part. Contacts

in all

circuit breakers

are

designed

so

that high currents

aid

in

forcing

the

contacts apart. Therefore, they

may

have

a

withstand rating below their interrupting rating.

Electronic trip units

are now

available

for

molded-

case circuit breakers.

These

are

considerably more

expensive than their thermal-magnetic counterparts,

but

they

are

available with

a

full

range

of

features that

increase pumping station reliability, because they

can

be

adjusted

to

coordinate with other breakers

in the

circuit. Hence,

a

serious short circuit

in a

branch cir-

cuit

would cause only

the

branch circuit breaker

to

trip,

thereby sparing

the

main circuit breaker

and

per-

Figure

8-14.

Operation

of

circuit

breakers,

(a)

Thermal

action;

(b)

magnetic

action.

For

both

types,

the

condition

of

the

breaker before

being

tripped

is at the

top.

mitting

the

pumping station

to

continue

in

operation.

Main

and

feeder thermal-magnetic circuit breakers

do

not

coordinate with each other

or

with branch circuit

breakers.

Molded-case

circuit breakers have several ratings.

Among

them are:

•

Rated current: most breakers carry

80% of

rated

current

continuously

(at

least

8

hours).

Refer

to

manufacturer's

literature,

and do not

specify

a

breaker rated below

15 A. Use

fuses.

•

Voltage:

240 or

480:

480 V

units

may be

applied

on

240 V

systems. Single-pole breakers

are

rated

120

or 277 V.

•

Interrupting rating (RMS Symmetrical):

a

circuit

breaker must

be

able

to

interrupt

the

maximum

fault

current

available

to it.

Operators should always

open

and

inspect

a

circuit breaker

after

such

an

operation, because

it may

need replacement.

Standard

thermal-magnetic circuit breaker ratings

are

given

in

Table

8-1.

Because

the

utility

may

increase

the

available

fault

current without notice, designers should

use

discretion

in

circuit breaker selection.

Do not

apply those subject

to

utility

fault

current near their interrupting rating,

and

allow

for

motor contribution

(4

times

full-load

current

for

common induction motors).

Most smaller

molded-case

circuit breakers

are

designed

for use in

circuit breaker panelboards.

A

cir-

cuit

breaker panelboard

is a

factory

assembly

of

cir-

cuit-breaker mounting arrangements

and

connecting

buses,

and may

contain

up to 42

single-pole circuit

breakers.

A

main circuit-breaker

may be

included.

The

enclosure

is of

sheet metal

for

safety,

and may

contain

a

door (sometimes lockable) over

the

circuit-breaker

operating handles. Circuit breakers

may be

either

plugged into

or

bolted

to the

buses. Bolting

is

recom-

mended

for

reliability,

but

bolting

is

somewhat more

expensive.

Circuit breakers

may be

provided with

a

number

of

accessories, including

lock-ons,

padlock hasps,

and

alarm contacts.

Refer

to the

manufacturer's literature.

Ground

fault

relaying

is

available

for use

with main

circuit breakers

and

with

electronic

trip units.

Low-voltage power circuit breakers

are

used

in

switchboards

as the

main

and tie

circuit breakers.

They

may

have trip ratings

to

4000

A or

higher. With-

stand

and

interrupting ratings

are

quite high,

and

they

may

be

obtained with back-up

fuses

for use up to

200,000

A.

Modern trip units

are

solid state with

a

full

range

of

adjustments,

and

ground

fault

relaying

is

available. Breaker contacts

are

opened

and

closed

by a

stored energy system (springs)

to

provide quick-make,

quick-break operation. Springs

are

compressed

by

electric motor devices.

In the

event

of

motor failure,

springs

may be

compressed manually. Remote electri-

cal

opening

and

closing control

is

possible, such

as for

switching

to

alternate

electrical

sources

or

standby

generators

and

tie-breaker operation. These breakers

are

easily inspected, maintained,

and

serviced.

Table

8-1. Standard

Thermal-Magnetic

Circuit

Breaker Ratings

Continuous

current

rating

Interrupting

rating

Continuous

current

rating

Interrupting

rating

120/240

V

277/480

V

15-100

A

10,00OA

15-100

A

14,00OA

15-150

A

22,00OA

15-125

A

18,00OA

100-225

A

42,00OA

15-100

A

25,00OA

15-30

A

65,00OA

70-250

A

25,00OA

125-400

A

30,00OA

300-1000

A

30,00OA

15-125

A

35,00OA

70-250

A

35,00OA

125-400

A

35,00OA

15-125

A

65,00OA

100-250

A

65,00OA

300-2000

A

65,00OA

600-1200

A

100,00OA

600-2500

A

100,00OA

Motor

Branch

Circuits

and

Controllers

The

requirements

for

each component

of a

motor

branch circuit

are

delineated

in

Article

430 of the

NEC.

A

single-line diagram

of a

typical motor branch

circuit

is

illustrated

in

Figure 8-15.

The

feeder

or

ser-

vice

disconnect

and

overcurrent

protective

device(s)

and

the

feeder

or

service conductors

are not

part

of the

motor branch circuit,

but

their size calculations

are

influenced

by the

rules

in

Article 430.

The

branch circuit disconnect

and

branch circuit

overcurrent protection consist

of

either

a

circuit

breaker

or a

fused

switch.

It

must

be

convenient

for a

worker

to

padlock

a

switch

in

open position

for

safe

motor

or

pump maintenance. Circuit breakers

in

motor control centers

are

often

adjustable magnetic-

only

devices,

often

called "motor circuit protectors."

They must

be

adjusted

high enough

to not

trip

on

starting

but to

trip

on a

short circuit. Fuses

are

some-

times dual-element types selected

for

both branch cir-

cuit

and

motor overload protection. Fuses

are

low-

cost, easily replaced devices that

are

fail-safe

and

eas-

ily

coordinated with other protective devices.

But

they

can

allow single-phasing

of the

motor. Some

fused

switches

are

arranged

to

open

the

switch automati-

cally

if a

fuse

opens. Fused switches must

be

horse-

power-rated below

200 hp, and

equivalent measures

must

be

taken above

200 hp.

Large, important motors,

particularly

in

sewage

lift

stations,

justify

the

cost

of a

Feeder

or

service

overcurrent protection

Feeder

or

service

conductor

Branch

circuit

disconnect

Branch

circuit

overcurrent

protection

Controller,

motor

control circuits

Running

overload

protection

Branch

circuit

conductors

Motor

Figure

8-15.

A

typical

motor branch

circuit

diagram

per

NEC.

single-phase protective relay

or of a

ground

fault

pro-

tective relay. Single-phase monitors

and

protection

devices

cost about

$700

per

motor

at

1997

prices,

regardless

of

motor size. Installation would

increase

the

cost

by

perhaps 50%. Where power

is

reliable,

the

frequency

of a

single-phasing event might

be

less than

once

per 20

years, whereas

it

might

be

once

per

decade

in

rural communities

or

where lightning

is

prevalent. Designers should assess

the

likelihood

of

burned-out motors

and the

loss

of

pumping against

the

cost

of

protection

to

balance

risk with cost.

Motor circuit conductors must

be

selected

to

carry

at

least 125%

of

motor-rated full-load current.

Controllers

for

AC

Squirrel-Cage Motors

The

controller must

be

capable

of

starting

and

stop-

ping

the

motor

and of

interrupting

the

locked rotor

current.

For

portable motors

l

/

3

hp and

smaller,

a

cord

and

plug assembly

may

serve

as the

controller,

For

stationary motors

l

/

s

hp and

smaller that

are

normally

left

running

and

cannot

be

damaged

if

stalled

or

jammed,

the

branch circuit protective device

may

serve

as the

controller.

For

attended motors, manually

operated motor starters

and

some light switches

of

suitable ratings

are

available

in

single-

and

three-pole

models,

For

unattended

or

automatically controlled

motors, motor starters using magnetically operated

contactors must

be

installed.

A

contactor

is a

heavy-duty solenoid-operated

switch used primarily

as a

controller

for

motor

on-off

control,

but

also

for

heaters

and

lights. When com-

bined with

an

overload relay,

the

assembly

is

known

as

a

"motor

starter."

A

motor starter with

a

circuit

breaker

or

fused

switch

is

known

as a

"combination

motor

starter."

A

motor contactor must

be

able

to

interrupt

locked-rotor

current. Locked-rotor current

is

drawn

during

the

starting period

or

indefinitely

should

the

pump

or

other driven device

be

jammed

or

stalled.

Locked-rotor current

is

roughly

six

times

full-load

current

or

more

for the

high-efficiency line

of

motors.

Contactors

are not

required

to

interrupt

fault

currents.

The

fuses

or

circuit breaker perform that

function.

The

locked-rotor current rating determines

the

horsepower

rating

of the

contactor.

Running

overload protection

is

intended

to

protect

motors, motor controllers,

and

motor branch circuit

conductors

from

overheating

due to

motor mechanical

overloads

and

from

failure

to

start.

It

does

not

protect

against short circuits

and

ground

faults.

Most

of the

common single-phase motors larger

than

VIS

kW

(V

2

Q

hp)

have

an

integral thermal switch

(in

series

with

the

motor

conductors)

that will shut

off

the

motor

if it

becomes overheated. Most three-phase

motors

are

protected

by

thermal

or

solid-state over-

load relays (one

in

each

phase)

that

are

part

of the

motor starter.

The

following description applies

to

relays

on

size

5

motor starters

and

smaller.

Thermal

overload relays

of the

melting alloy

and

bimetal types have been available

for

many years.

Connected

in

series

with motor conductors

and on the

load side

of the

contactor, they carry motor current

and

are

heated thereby. Should excessive current

flow for a

length

of

time

due to

mechanical overload

or to a

locked-rotor

condition,

the

relay (connected

so as to

de-energize

the

contactor coil when hot)

is

gradually

heated.

As the

motor does

not

heat instantaneously,

the

relays

are

designed

to

have

a

time

lag

matching that

of

the

motor,

and

this time

lag

allows

the

motor

to

start.

Relays

are

available

in

quick trip (Class 10), standard

trip (Class 20),

and

slow trip (Class

30)

versions,

and

can be set for

manual

or

automatic reset. Automatic

reset

should

be

selected

for

unattended pumping sta-

tions.

Because ambient temperatures vary

and

signifi-

cantly

affect

motor heating, noncompensated relays

are

provided

as

standard. Ambient-compensated relays

are

available

for

motors

in a

constant ambient condi-

tion, such

as in a

submerged application. Relays have

heaters selected

to

match

the

motor

and

other condi-

tions,

and may be

adjusted

within limits

as

necessary

for

a

closer match

to

motor-heating characteristics.

Solid-state overload relays

are now

available

as

direct replacements

for the

melting alloy type. They

are

somewhat more expensive than thermal relays.

These solid-state relays have

a

large adjustment range

and

so do not

need

heaters.

They

may

have other fea-

tures, such

as

phase loss protection, phase unbalance

protection,

and

electrical

remote reset.

Across-the-Line

Starters

The

across-the-line

(or

full

voltage) starter

is the

most

commonly used starter

in

pumping stations because

of

its

simplicity,

ease

of

maintenance,

and

relatively

low

cost.

It is

nearly always chosen

for

motors

of 55 kW

(75 hp) and

smaller. Motor starters

are

available

in the

following

styles:

•

Full

Voltage Non-Reversing (FVNR)

or

"across-

the-line"

•

Full Voltage Reversing (FVR)

•

Reduced Voltage Non-Reversing

(RVNR).

Most

of the

above styles

of

motor starters

are

avail-

able

in

low-voltage starters (below

600 V) and in

medium-voltage starters

(2300

to

13,200

V). The

rat-

ings

of

motor starters

are

given

in

Table 8-2.

No

motor starter smaller than Size

1

should

be

specified

or

permitted

in

pumping stations,

and

some

designers

say

Size

2. The

cost

difference

is not

great,

but

the

larger starters

are

considerably more robust

and

ensure greater

reliability

and

longer service

life.

Use

caution

in

specifying motor starters. Thorough

knowledge

of the

motor-operating conditions

and

Table 8-2.

Motor

Starter Ratings

Maximum

hp

Maximum

hp

NEMA

size

System

voltage

Single phase Three phase

NEMA

size System

voltage

Three phase

OO

120

V

2

3 208 25

208

IV

2

240 30

240 1

IV

2

480 50

480 2

0

120 1 4 208 40

208

3 240 50

240

2 3 480 100

480 5

1 120 2 5 208 75

208

7V

2

240 100

240

3

7V

2

480 200

480 10

2

120 3 6 208 150

208 10 240 200

240

7V

2

15 480 400

480 25

7

240 300

480

600

other constraints

is

required

for

selecting starter

equipment

for

each application.

Prices

vary widely,

so

designers must

not

specify

an

expensive

soft

starter

when

a

simple

across-the-line

unit would

be

adequate,

nor

should

an

overly expensive starter

be

specified

because

of a

lack

of

design information.

The

decision

should

be

made

after

sufficient

information

has

been

developed relating

to the

load conditions, accelerating

time

of the

drive,

and

accelerating time allowed

by the

serving

utility.

Also,

in

pumping applications, avoid

simple

on-off

controls

for

pumps when

the

pumping

cycle time

is

likely

to be

very short (more than

3

starts/min).

Otherwise, derate motor starters

by

using

their jogging horsepower rating (see manufacturer's

literature). Starting

too

frequently

causes motors

and

motor starters

to

overheat

due to the

high inrush cur-

rent. There have been situations where too-frequent

starts

have caused early

failure

of a

motor. Failure

is

even

more likely

to

occur

if the

motor

is

undersized

for

the

peak load during

the

short-duration pumping

periods, because both

the

load

and the

starting heating

effects

are

additive.

Reduced-Voltage Starters

Use of a

reduced-

voltage

starter should

be

considered:

(1)

if the

power system cannot provide

the

high start-

ing

currents typical

of

larger motors,

(2) if the

power

company

limits starting current

or

kVA,

(3) if a

standby

generator

is

used,

(4) if the

feeder and/or

branch circuit

is

longer than

18Om

(600 ft),

or (5) if

the

voltage drop

due to

starting current would

be 20%

or

more. Actually,

the

motor would start

and run at a

30%

drop,

but the

contactors might drop out.

When

a

motor

is

started

at

reduced voltage,

the

current

to the

motor

is

reduced

in

direct proportion

to

the

voltage reduction,

and the

torque

is

reduced

by the

square

of the

voltage reduction.

If a

"typical"

NEMA

B

motor

is

started

at 70% of

line voltage,

the

starting

current would

be 70% of the

full

voltage value

(i.e.,

0.70

x

600%

=

420%).

The

torque would

be

then

(0.7O)

2

or 49% of the

normal starting torque (i.e.,

0.49

x

150%

= 74% of

full-load

torque).

In

pumping

stations,

reduced-

voltage

starting

effectively

reduces

inrush

current, while

the

lower starting torque

may

pose

no

problem.

Autotransformer

starters

are the

most popular

in

pumping stations. Refer

to

manufac-

turers'

literature

for

details

of

construction

and

opera-

tion

of

each type,

and to

Table

8-3 for

their features,

advantages,

and

disadvantages.

Motor

Control

Circuits

and

Devices

The

simplest, most commonly used motor control cir-

cuit

utilizes momentary start-stop push buttons

and is

illustrated

in

Figure 8-16. This control circuit

is

some-

times

called

"three-wire"

control because three con-

trol conductors

are run

from

the

motor starter

to the

pushbutton

station.

Momentarily depressing

the

start

button

energizes

the

contactor

M

coil,

which

is

sealed

in

through

the

normally

open

M

contact (which

closes

when

the

contactor main contacts

close).

The

start

button

may

then

be

released without dropping

out the

Table

8-3.

Reduced-Voltage

Starters

for

Motors

Type

Advantages

Disadvantages

"Soft

start,"

solid state

Low

starting line current. Most expensive.

Frequent starting capability. Sensitive

to

line transients, power quality,

and

high ambient

temperatures.

Autotransformer

High torque. Expensive.

Low

starting line current.

Adjustable

taps

for

reduced voltage.

Primary

resistor Multiple accelerating points. Expensive.

Frequent

starting capability. Resistors generate heat.

High

power

factor.

Starting time less than

5

seconds.

Wye-delta

High torque

and

efficiency.

Long acceleration starting time.

Moderately

priced.

Low

starting torque.

Low

starting current. Motor must

be

suitable.

Part

winding

Compact.

Low

starting torque.

Least expensive. Starting time less than

2

seconds.

Not

suitable

for

frequent

starts.

Nonadjustable.

Motor must

be

suitable.

contactor,

and

thus

the

motor

is

energized through

the

closed main contacts

of the

starter. Depressing

the

stop button

or

tripping

an

overload relay de-energizes

the

M

coil,

opens

the

line contactor,

and

thus discon-

nects

the

motor

from

the

line.

A

similar control circuit using

a

maintained

(a

con-

tact

that remains closed

after

actuation) two-position

(On-Off)

or a

three-position

(On-Off-Auto)

selector

control switch

can

also

be

utilized

for

motor starting

and

running control.

See

Figure

8-17.

The

selector

switch

control circuit allows

the

motor

to

operate

in

either

the

manual

or

automatic mode.

The

manual

mode consists

of the On and

Off

positions.

In the

Auto

mode (with

the

selector

in the

Auto position),

the

motor

is

started

or

stopped

by the

closing

or

opening

of,

for

example,

the wet

well level control switch

located remotely

from

the

motor controller.

Control circuits

for

pumping applications

may be

designed

to

meet other requirements, such

as:

•

Pump sequencing, which reduces

the

frequency

of

motor starts, exercises rotating equipment

and

Figure

8-16.

Pushbutton

three-wire motor control circuit.

Figure

8-17.

Three-position

selector

switch motor control circuit.

alternates

the

pumps

to

start

on

each successive

operation.

The

pumps, however, tend

to

wear

out at

about

the

same time, which

may or may not be

acceptable

to the

owner.

• A

time delay, which

is

used

in a

control circuit

to

delay

the

restarting

of the

pump

after

the

restoration

of

power following power failure.

• A

permissive condition that must

be

satisfied

before

control action

can

become effective, such

as

delay

in

shutting down

an

operating pumping unit until

the

replacement

unit

has

reached

a

specific

speed

range

and is

thus capable

of

taking over

the

pump-

ing

duty.

•

Electrical interlocks, which

are

usually

a

series

of

contacts

on

relays

or

other control devices used

to

enable

or

prevent

the

operation

of

equipment

in a

prearranged sequence.

Motor

Control Centers

A

motor control center (MCC)

is a

unitized assembly

of

grouped motor controllers, interconnecting buses,

wiring

spaces,

and

other

electrical

controls

and

equip-

ment

associated

with

providing power

and

control

for

several motors. Motor control centers provide incom-

ing

line facilities, main circuit breaker space

(if

needed), horizontal bus,

and a

vertical

bus in

each sec-

tion.

MCCs

may be

obtained with plug-in cubicles

for

NEMA

Size

1

through Size

4

motor controllers. Sev-

eral

classes

of

control wiring arrangements

are

identi-

fied

by

NEMA standards. Layouts

of

typical MCCs

are

shown

in

Figures 8-18

and

8-19.

The MCC

struc-

ture

must have

an

enclosure

and a

seismic rating com-

patible

with

the

location

in

which

it is

installed.

The

built-up

motor control panel shown

in

Figure

8-19c

is

a

low-cost alternative

to an

MCC

—

but

only

if

labor

costs

are not

considered.

It

is

important that

the

motor

control

center

(bus

bracing, combination starters,

and

individual circuit

breakers)

be

rated

for the

short circuit duty predicted

by

the

fault

calculations.

If a

main circuit breaker

is to

be

provided

in the

unit,

then

its

setting should

be

coor-

dinated

with

the

supply

overcurrent

equipment

as

well

as

with

the

branch circuit breakers connected

to the

buswork.

In the

coordination with

the

branch circuit

breakers feeding

a

number

of

motors,

the

breaker

feeding

the

smallest motor

has the

highest interrupting

duty,

so be

certain that protection

for the

smallest

motor

has

adequate interrupting capacity.

The

reason

for

this high interrupting duty

on the

smallest circuit

is

that during

the

initial period

of the

short circuit,

all

other

running

motors contribute current

to the

fault,

thus

adding

to the

short-circuit current

from

the

main

Figure

8-18.

Elevation

view

of a

typical

low-voltage

motor

control

center.

Cubicles

are

labeled

with

name

of

load.

Permanent

name

plates

are

recommended.

power

source. Large motors contribute larger short-

circuit currents than

do

small motors.

NEMA Class

II,

Type

B

wiring

is

recommended

in

the

MCC.

In

this wiring method, factory power

and

control wiring

is

terminated

on

terminal blocks

in or

adjacent

to the

cubicle.

The

terminal blocks

form

the

interface

between factory

and field

wiring.

The

fac-

tory

furnishes

complete layout, wiring,

and

control

diagrams

for

each unit

as

well

as for the

entire assem-

bly.

All

terminals

are

labeled

and

each wire

is

num-

bered and/or color coded.

The

manufacturer's

diagrams should

be

required

to

include

the

entire

motor branch circuit

and

controls.

It

is

advisable

to

require:

(1) the

circuit breaker

(circuit disconnect) handle

of

each unit

to be

lockable

in

the

open position

and (2)

provisions

for at

least

two

padlocks

to be

used

for

locking

out the

circuit.

The

door must

be

interlocked

so

that

the

circuit breaker

is

in the

OfY

position before

the

door

of the

unit

can be

opened.

There must

be

enough space

in

each cubicle

so

that

no

device

is

mounted behind another device.

All

con-

trol equipment must

be

accessible

for

inspection when

the

front

door

of the

cubicle

is

open. Empty spaces

or

spaces wired

for

"future"

starters must have

bus

open-

ings completely covered

so

that when

the

door

is

open,

no

live buses

are

exposed.

Motor control

centers

should

be

installed

in

easily

accessible locations with adequate spacing

from

other

equipment,

in

accordance

with

NEC and

local

and

state

codes. They should

be in a

clean,

dry

location

to

avoid

the

need

for

specifying

the

more expensive gas-

keted

enclosures.

The

space should

be

well lighted

and

ventilated (see Chapters

9 and

23).

Where control power transformers with

fused

primary

and/or secondary

are

provided,

it is

advisable

to

install

a

blown-fuse indicator

in the

door

of

each

unit

for

monitoring

the

control

cir-

cuit

continuity.

It may

also

be

desirable

in

some

installations

to

include door-mounted pushbutton

controls

for the

motor

and

indicating lamps

to

show:

(1)

power available,

(2)

motor running,

(3)

automatic

or

remote control indicators,

and (4)

running-time indicator.

In

large stations where

computer control

and

monitoring

is

used,

the

run-

ning-time

records

may be

more economically

kept

in the

computer

files.

Sometimes

the

plant control panel

is

installed

in

the

motor control center when there

is no

other suit-

able location.

Figure

8-19. Examples

of

different

types

of

motor

control

centers,

(a)

Single-line

diagram;

(b)

motor

control

center;

(c)

built-up

motor

control

panel.

Insulated

Cables

and

Conductors

Cables

and

conductors

are not the

same thing.

A

cable

contains

two or

more conductors.

Cables

Insulated conductors used

in

electric

circuits

may be

grouped together

and

surrounded

by a

jacket

to

form

an

insulated

cable. Cable jackets must

be

suitable

for the

location

and

application, because they

may be

exposed

to

moisture, dirt, etc., without

the

protection

of

conduit.

Conductors

and

Terminations

Most conductors

are of

copper

or

aluminum. Both

metals

are

considered

to be

excellent conductors

of

electricity.

The

conductivity

of

aluminum

is

only

61%

of

that

of

copper,

with

the

result that aluminum con-

ductors

are

approximately

one

size larger than

the

equivalent

copper conductor.

The

specific

gravity

of

aluminum

is 30% of

that

of

copper,

so

aluminum con-

ductors

are

lighter

and

easier

to

handle.

Conductors

may be

solid

or

stranded.

The NEC

requires that conductors

No. 8 AWG and

larger

be

stranded.

Smaller conductors

may be

either solid

or

stranded.

A

number

of

types

of

stranding

are

avail-

able,

and the

selection depends upon

the

application.

Conductors

No. 2 AWG and

smaller

are

usually

7-

strand

rope-laid

(center strand surrounded

by 6

outer

strands).

In

larger sizes,

12

strands surround

the

basic

7 to

form

a

19-strand

conductor.

Oxide

forms

on the

surface

of

both copper

and

alu-

minum

—

slowly

on

copper

and

rapidly

on

aluminum.

Copper oxide

is a

relatively good conductor,

but

alumi-

num

oxide

has

high resistance

and

must

be

removed

in

the

termination process

to

prevent termination over-

heating

and

failure.

Conductor termination workmanship must always

be of the

highest quality

to

ensure

a

reliable, trouble-

free

electric system.

But

aluminum terminations

are

much

more sensitive

to

workmanship than

are

copper

ones.

The

aluminum must

not be

nicked when

the

insulation

is cut

off.

The

aluminum oxide must

be

removed

by

wire brushing,

and the

aluminum must

be

immediately

coated with

a

conductive anti-oxidant

compound.

The

connection must

be

torqued

to

manu-

facturer's

specifications,

retightened

the

next day,

and

retightened annually, because aluminum

has a 31%

greater thermal

coefficient

of

expansion, which,

together with

its

tendency

to

creep, loosens connec-

tions (see

ANSI/AIEE

141).

The

only

reliable

alterna-

tive

to

torquing bolted terminals

is the use of

compression lugs,

and

these

are

recommended

for

high-quality

work

for

both copper

and

aluminum con-

ductors

in

Size

No. 2 AWG and

larger. Terminals

for

use

with aluminum conductors must

be so

labeled.

Because

of the

termination problems,

it is

industry

consensus that

only

copper

conductors should ever

be

specified

and

used

in

pumping stations,

Buses

are

electric

conductors

in the

form

of

bars

of

various shapes

and

sizes. Buses

are

used

in

motor con-

trol centers, switchgear,

and

elsewhere

for

high

amperage capacity. They

may be

insulated

for

safety.

Aluminum

buses

are

often

tin-plated

for

corrosion

resistance. Bolted connections

are

made with

belleville washers, properly torqued.

Insulation

Most insulating material

is

composed

of

organic com-

pounds.

Two

exceptions

are

magnesium oxide, used

for

mineral-insulated cables,

and

mica, used

in

some

high-voltage cable construction. Insulation

and

cable

jackets

for

pumping station applications should have

physical

and

electrical

characteristics such

as:

•

Resistance

to

moisture

•

Resistance

to

heat

•

Resistance

to

ozone

•

High dielectric characteristics

•

Resistance

to

abrasion

•

Hardness

at

utilization temperature.

Examples

of low

voltage (600

V)

conductor insulation

are

given

in

Table 8-4.

Medium-voltage

(5 to 15 kV)

cables

may be

con-

structed with semiconducting tapes

on the

inside

and

outside

of the

insulation with

a

conducting shield

between

the

outer semiconducting tape

and the

jacket.

Medium-

voltage

cable insulation materials commonly

used are:

•

Natural rubber

•

Butyl rubber

•

Cross-linked polyethylene (XLPE)

•

Ethylene-propylene (EPR).

Shielded cables should always

be

used.

The use of

shielding

in

medium-

voltage

cables lowers

the

electrical

stress

on the

cable insulation

by

providing

a

grounded

surface

equidistant radially

in all

directions

from

the

con-

ductor. When laid

on a

metallic

surface,

an

unshielded

cable allows

a

distorted

field

pattern within

the

cable

structure

that causes

a

high voltage gradient

in a

portion

of

the

insulation.

It

shortens

the

life

of the

cable.

Current Rating

The

ampacity (current rating) depends upon

the

con-

ductor material, size, type

of

insulation,

and the

ambi-

ent

conditions

of its

installation. Refer

to NEC

Tables

310-16

through

310-19

and

310-69

through

310-84

and

the

notes pertaining

to

them.

Raceways

and

Wireways

The NEC

requires that

all

electrical conductors

be

enclosed

and

protected

by

some

form

of

raceway.

Jacketed cables, where permitted,

are

exempt.

The

most

common

forms

of

raceways

in

pumping stations

are (1)

conduit

and (2)

wireways.

Galvanized

rigid

steel

is the

most commonly used

material

for

conduit systems. Lighter grades than

schedule

40

steel

do not

generally exhibit

the

degree

of

corrosion resistance required

for

long

life

in the

damp

surroundings

that

are

common

in

pumping stations.

Threaded connections should always

be

used.

The

specifications

should require

full

hot-dip galvanizing

treatment

after

fabrication

and

threading. Field-cut

threads

should

be

painted with

a

zinc

-bearing

material.

On

small projects,

if

high-grade hot-dip galvanized

conduit

is not

available, consider schedule

40 or 80

rigid PVC

conduit

in wet

places

and

where concrete

encasement

is

required.

If the

pumping station

is

part

of

a

large project, wherein

office

and

shop

space

is

allocated, intermediate-grade galvanized conduit

may

be

specified

for

these areas.

On all

projects where

PVC

conduit

is run

through

or

embedded

in

concrete,

special precautions must

be

taken

to

avoid breakage

during

the

placement

of the

concrete.

Wireways

are

square

or

rectangular metal channels

or

troughs with removable covers designed

for

routing

and

connecting conductors.

In

pumping stations,

use

wireways

in dry

locations only.

The

main advantage

of

wireways

is the

ease

of

access

for

additions

or

removal

of

conductors

and in

adding circuits.

A new

conduit

may be

readily connected

to a

raceway

and

new

power

and

control circuits added

at

minor cost

compared

to the

cost

of a new

conduit

run to the

near-

est

power source.

If

the

mechanical specifications

for the

project

require painting

of all

mechanical equipment

and

exposed metal

in a

particular area

of the

station, then

the

exposed conduit

and

wireways should

be

painted

to

conform

with

the

other exposed metal.

The

zinc coat-

ing

on

galvanized metal must

be

properly pretreated

and

primed before painting.

Any

conduit entering

a

hazardous location must

be

provided with

an

explo-

sion-proof seal

fitting filled

with

a

UL-listed

sealing

compound

after

all

conductors

are

installed. Conduit

runs

must

be

sloped

to

drain,

and

breather/drain

fit-

tings

(or,

in

nonhazardous

areas, weep holes) should

be

provided

at any low

point

in a run and

prior

to

entering

control cabinets, MCCs,

and

control panels.

All

conduits below grade, whether buried, embed-

ded

in

concrete,

or

exposed

in

underground rooms

or

spaces,

are

subject

to

infiltration

of

water

and

should

be

sealed inside

the

conduit around

the

conductors

and/or cable(s)

at

each entrance into

any

enclosure

below grade. O-Z/Gedney

[1] has

excellent sealing

bushings

and fittings

that utilize sealing glands

tight-

ened

by

accessible

set-screws that

can be

retightened

during

maintenance

if

necessary.

All

conduits

or

cables entering

a

normally

dry

underground

room

or

space

in a

pumping station

and

originating outside (buried)

or in a

room

or

space sub-

ject

to

possible

flooding

(such

as a wet or

clear well)

should

be

installed

in

through-wall

or floor

sleeves

equipped with sealing glands tightened

by

accessible

set-screws that

may be

retightened. O-Z/Gedney

[1]

has

suitable sealing sleeves.

Junction

and

Outlet Boxes

Junction

and

outlet boxes installed

in

pumping stations

should

be

hot-dip galvanized cast iron. They should

be

surface-mounted,

spaced

a

minimum

of 6 mm

(

1

I

4

in.)

from

walls

or

mounting plates,

and

placed

as

high

as

possible

in dry

wells. Many pumping stations

are

subject

Table

8-4.

Insulation

Types

for

Low-Voltage

Electrical

Conductors

Maximum operating

Type

Insulating material

temperature,

0

C

a

Application

RHH

Heat-resistant rubber

90 Dry and

damp

RHW

Moisture-

and

heat-resistant rubber

75 Dry and wet

THW

Moisture-

and

heat-resistant

thermoplastic

75 Dry and wet

THHN Heat-resistant thermoplastic

90 Dry and

damp

THWN Moisture-

and

heat-resistant thermoplastic,

poly

amid

75 Dry and wet

nylon

jacketed

XHHW Moisture-

and

heat-resistant

cross-linked

polymer

75 Dry and wet

90 Dry and

damp

a

Can be

operated

at

these temperatures

only

if the

circuit protective device

is

tested

and

rated

by UL at

these temperatures.

Pumping Station Desing - Second Edition by Robert L. Sanks, George Tchobahoglous, Garr M. Jones

Подождите немного. Документ загружается.