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Draw-out breakers an d contactors were

c o n c e i v e d p r i m a r i l y b y t h e s a m e

com pan ies who also designed an d supplied

m eta l c lad a nd m e ta l enc losed swi tchgea r

t o u t i l i t i e s a n d h e a v y i n d u s t r i a l u s e r s

during the early 1940’s. These draw-out

con tac to rs and b reake rs were in t roduced

as the next generation of control af ter oil

i m m e r s e d b r e a k e r s . B e c a u s e o f t h e i r

p h y s i c a l s i z e a n d t h e r e q u i r e m e n t s f o r

r e g u l a r r o u t i n e m a i n t e n a n c e , a i r b r e a k

c o n t a c t o r s a n d c i r c u i t b r e a k e r s w e r e

des igned to be “d rawn-ou t” o r “ racked-

ou t . ” T he rack ing ou t o f the con tac to r o r

breaker generally provided the only m ean s

fo r iso la t ing the dev ice and the rem ainder

o f the b ranch c i rcu i t from the m a in power

bus .

T h e u s e o f d r a w - o u t c o n t a c t o r s a n d

breakers in t roduced severa l p rob lema t ic

issues:

1 . T h e a d d it io n o f sl id e- o n or “ fi n g er

style” pressure con tacts added several

p o i n t s o f c o n n e c t i o n t h a t , i f n o t

p r o p e r l y m a i n t a i n e d , c o u l d c a u s e

even tua l con tac t f inge r fa i lu res (h o t

s p o t s ) d u e t o t h e t h e r m a l c y c l i n g

( e x p a n s i o n a n d c o n t r a c t io n o f t h e

m e t a l ) o f s t a r t i n g a n d s t o p p i n g t h e

load . T hese types o f fa i lu res , i f the

equ ipm en t i s n o t rou t ine ly in spected ,

a r e n o t a l w a y s v i s i b l e u n t i l

catastrophic equipm ent failure occurs.

This is , a lm ost a lways, the weak l ink

in th e power circuit .

2 . O n so m e dr a w- ou t co n ta c to r t ra y

assembl ie s , where the re i s no o the r

isolation means, i t is possible to rack

in th e draw-ou t tray assem bly directly

in to a fau l t cond i t ion .

3 . M ec h a n ic a l dr a w- o u t tr a y p osi ti on

indicators add addit iona l e lem ents that

can sub ject the equ ipm en t to po ten t ia l

nu isance t r ip s.

In the 1950’s , the o r ig ina l Rockwel l

Au tom at ion /Al len -Brad ley 400 Am p Air

Break Contactor (see Figure 1)  was alsodesigned in a dra w-out s tyle to satisfy the

n e e d fo r t h e r e gu l a r m a i n t e n a n c e a n d

inspec tion o f the con tac to r a rc chu tes an d

m ain power conta cts. A draw-ou t assem bly

was also required becau se the sheer weight

a n d s i ze o f t h e a i r b r e a k c o n t a c t o r

p r e v e n t e d a c c e s s t o o t h e r p o w e r c e l l

com ponen ts beh ind i t . I so la t ion f rom the

main power bus was p rov ided by a non-

load b reak i so la t ion switch s im i la r to the

s wi t ch c u r r e n t l y i n c o r p o r a t e d o n a l l

Al len -Brad ley Med ium Vol tage S ta r te r s

toda y. This switch pr ovides tru e an d visible

isolation of the power cell when the switch

is opened.

T h e r e i s a m i n d s e t b y s o m e u s e r s ,

however, that a l l conta ctors mu st be draw-

out to insure isolation from the power bus.

T h i s i s o n l y t r u e w h e n i t c o m e s t o

Draw-out Versus 

Fixed Mounted Contactors 

Figure 1: Airbreak Contactor 

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Technical Data 

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switchgea r s tyle circuit break ers and som e

of our com petitors’ s tarters , where th ey do

n o t s u p p l y a n y o t h e r w a y t o p r o v i d e

i s o l a t i o n f r o m t h e p o w e r b u s . T h i s

ph ilosophy has been rein forced with

users of competit ive equipm ent where

d raw-ou t s ty le con tac to rs a re s t i l l

u s e d - e v e n o n v a c u u m s t y l e

c o n t a c t o r s . H o w e v e r , t h e

A l l e n - B r a d l e y , n o n - l o a d b r e a k

isolation switch (see Figure 2) , in

c o m b i n a t i on w it h a m e d iu m vo l ta g e

vacuum contactor , provides a safer ,

m ore visible m ean s of isolation from

t h e m a i n p o w e r b u s .

W i t h t h e d a w n i n g o f v a c u u m

techn ology, the need to withdraw th e

con tac to r fo r m a in tenan ce was m ade

u n n e c e s s a r y ( n o a r c c h u t e s o r c o n t a c t

i n s p e c t i o n ) . T h e v a c u u m c o n t a c t o r s o f  

today (see Figure 3)   require vir tually no

p h y s i c a l m a i n t e n a n c e a n d a r e m u c h

sma l ler tha n th e o ld a i r b reak techn o logy.Also, the Allen-Bradley MV starter power

cell has been redesigned to allow a ccess to

a l l m a jo r power ce l l com ponen ts wi thou t

remov ing the con tac to r (see Figure 4) . I n

fac t , even the ma jo r componen ts o f the

contactor have been designed to be easily

rep laced ( vacuum bo t t le , ope ra t ing co i ls ,

an d au x i l ia ry con tac ts) wi th the con tac to r

in i ts f ixed posit ion . Also, even th ou gh the

con tac to r i s f ixed moun ted , i t has been

designed to be easily an d qu ickly rem oved,

if required.

Figure 2 : Isolation Swit ch 

(shown in closed position) 

Figure 3 : Vaccuum Contact or 

Figure 4 : Three-line Referenced to Actual 

Power Cell Layout 

R o c k w e l l A u t o m a t i o n c o n t i n u e s i t s

tradit ion of supplying a posit ively driven

n o n - l o a d b r e a k i s o l a t i o n s w i t c h a n d

va c u u m c o n t a c to r i n a l l m e d iu m vo l ta g e

starters . This design, which in corporates a

l igh t we igh t ye t indus t r ia l r a ted , f ixed

m o u n t e d v a c u u m c o n t a c t o r , s i m p l e

m e ch a n i ca l i n t e r lo c ks a n d a n o n - l o a d

break isolation switch, provides the u tm ost

of safety, practicality and reliability. With

b o l t e d c o n n e c t i o n s f r o m t h e i s o l a t i o n

s w i t c h c o m p l e t e l y t h r o u g h t o t h e

cus tomers load cab le conn ec tion po in ts,

t h e r e a r e n o c o n n e c t i o n s t h a t a r e

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Figure 5 : Symmetrical conductors and armored 

sheathing improves flux cancellation and 

reduces noise 

dependen t on spring loaded f ingers to pass

the fu l l r a ted cu r ren ts (wi th the excep t ion

of the clip connections for c l ip-on style

power fuses) . When the isolation switch isopen , the power ce l l a rea i s comple te ly

isola ted and ba r r ie red from an y m ed ium

voltage, thu s provides a totally “dead front”

power cell. The user is now free to verify

con tac t wear on th e vacuu m con tac to r o r

to wipe down o r vacuum any dus t f rom

the in te r io r o f the cab in e t and con tac to r.

An isolation switch viewing win dow is

provided in the power cell door (see Figure

5) to verify tha t the isolation switch is open

before the power cell door is opened. Also,

the m echan ica l in te r lock on the i so la t ion

swi tch p reven ts en te r ing the power ce l l

when the isolation switch is c losed (u nless

the in te r lock i s ma nu a l ly defea ted by the

use r to ga in access ) .

M o st d r a w - o u t v a c u u m c o n t a c to rassem bl ies a re la rge , awkward to ha nd le ,

a n d e x ce ed t h e m a x i m u m w ei g h t a l l ow ed

by OSHA for a s ingle in dividual to l if t or

c a r r y . M a n y c o m p e t i t i v e u n i t s a l s o

inco rpora te the m a in power fuses , con t ro l

c i rcu i t t r ans fo rmers an d the p r im ary fuses

for the con t ro l c i rcu i t t r ans fo rm er on th e

draw-ou t carr ia ge assem bly. A lift ing h oist

or trolley is thus requ ired to m ove, install

o r wi thd ra w the ac tua l ca r r iage a ssembly

from th e starter.

The m echan ical in terlocks for draw-out

carriage assemblies tend to be com plicated,

requ i re yea r ly se t-up a nd a d jus tmen t and

Figure 5 : Isolation Switch Viewing Window 

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   T    ∝    ∝    ∝    ∝    ∝

    V   2

Figure 7: Torque/Speed Relat ionship 

to Reduced Voltage Starting 

require periodic lubrica tion to insure their

safe operation . Man y suppliers rely solely

on gra vity to close the shu tter m echan isms

tha t provide isolation from live power buscomponen ts .

Rockwel l Au tomat ion recogn ized the

need fo r som e indus t r ia l u se rs to m a in ta in

consistency in their operating and safety

p rac t ices fo r a l l m ed ium an d h igh vol tage

e q u i p m e n t . S o m e u s e r s r eq u i r e t h a t a l l

con tac to rs o r b reake rs be wi thd rawn to

insu re tha t the power ce l l i s comple te ly

i s o l a t e d . B e c a u s e t h e r e i s n o n o n - l o a d

break i so la t ion swi tch in a ma in c i rcu i t

b r e a k e r c o m p a r t m e n t , t h e u s e r m u s t

withdraw the breaker to provide an d insure

isola t ion f rom the m a in power bus .

This need for consistency in operating

and safety practices, for some users, has

carried over into som e com petitive starters.

R o c k w e l l A u t o m a t i o n / A l l e n - B r a d l e y

ackn owledged this fact an d ha s developed

a d r a w - o u t s t y l e v a c u u m c o n t a c t o r

(see Figure 6) for those customers who sti l l

r e q u i r e t h i s c o n s i s t e n c y . T h e m e d i u mvoltage starter continues to uti l ize the non -

load break isolation switch bu t provides the

availabil i ty to the u ser to also withdra w the

con tac to r to ve r i fy comple te power ce l l

i s o l a t i o n . T h e R o c k w e l l A u t o m a t i o n /  

Allen-Bradley draw-ou t vacu um conta ctor

sti ll only weighs approxim ately 50 poun ds

(22 kg ) an d p rov ides the u ser the ease o f  

qu ick in te rchangeab i l i ty o f a con tac to r ,

shou ld the n eed a r i se .

O u r e x p e r i e n c e i n m a n u f a c t u r i n g

m ed ium vol tage sta r te r s for m ore than 60

years h as proven tha t the u se of a posit ively

d r iven , non- load b reak i so la t ion swi tch

a n d f ix e d m o u n t e d v a c u u m c o n t a c to r,

provides the u ser with the safest an d m ost

reliable approach to providing a dead front

power cell . I t is no t an issue o f providing

the custom er with th e safest , m ost reliablean d cost-effective solution (wh ich th e fixed

m oun ted con tac to r so lu t ion su re ly is ) bu t

ra the r a m a t te r o f in fo rming th e cus tom er

of the issues an d a ddressing the past history

tha t h as shaped a cus tomer’s pe rcep t ions

o f wha t i s accep tab le an d n o t accep tab le.

Figure 6 : Draw-out Contactor 

 Medium Voltage Products 135 Dundas Street, Cambridge, ON, N1R 5X1 Canada, Tel: (1) 519.740.4100, Fax: (1) 519.623.8930, www.ab.com/mvb

Publication 1500-TD029A-EN-E – January 2002 Copyright © 2001 Rockwell Automation. All rights reserved. Printed in Canada.