View Full Version : Star delta to softstart retrofit question

13-03-2012, 04:23 PM
Trying to find out if retrofitting as I,m about to describe is breaking any UK electrical regulations;

Have a compressor ( 310a FLA) on a start delta configuration; Intrestingly the L1 , L2 & L3 are fed via two 250 circuit breakers ( attached to the main buss bars) ; so the line & delta contactors are have their own phase conductors connecting to them ( although from the same supply).
I am going to replace the contactors with a soft start & was hoping not to have to replace the 2 circuit breakers with a larger , single circuit breaker. I was proposing to join, the L1, L2 & L3 from the 2 circuit breakers respectively ( in the terminal housing of the SS). SS is to be DOL in S/D configuration.

All the above are mounted behind panel doors with an interdependent main isolator.

My question is, it will work okay but is this breaking any electrical regs ( a colleague tells me it does but cant refer me to where this is written).
Thanks for any advise.


install monkey
13-03-2012, 08:20 PM
if your fitting a soft start-you shouldnt need 2x 3ph supplies as the starting current will be greatly reduced-whats the running current at full load?- if under 250amp then i would say its ok, better than using 2 breakers at 250a wired in parallel- the breakers are rated for protecting the cable

13-03-2012, 11:03 PM
A sketch is worth a thousand words.
The 2 x 250 amp breakers are there to protect motor in star delta configeration as each is only reading half the drawn current at motor running in delta. You will possibly have an overload after the Start contactor and it will be set at half or below FLA.
With 310 Full load amps and a soft starter you wil need a current protected isolator, [a D curve rated breaker is ok, but will be bigger than 250 amps ] before drive and the overload protection functions are programable in drive itself, I personally prefer Danfoss HVAC series drives and soft starters .

14-03-2012, 12:50 AM
Forgot to add, that the supply breaker/ current protect isolator switch is to be selected as a fault rating protection for system, and has to isolatable and be able to be locked out and tagged.
310 amps full load current is a large motor, double check the MCC fault rating on panel as well, with modifications the fault rating has to be brought up to current standards.

14-03-2012, 09:18 AM
Dear Sir,

I cannot direct you to specific documentation - unlike other publications, I do not read the Wiring Rules for enjoyment, only for money.

Two issues that I can identify are - multiple supply & cable protection.

The problem with multiple supply is that somebody could isolate one circuit breaker but not the other (& think the circuit is safe). Realistically it's not that much different to the current arrangement - suitable labeling should suffice.

With the cable protection, provided ALL the switchgear cables & motor leads can stand up to the 500 Amps thats ok. Depending on the soft starter terminal arrangement, if three wire output - the existing leads can probably be doubled up (two per phase). However, this is disregarding any motor protection. A conventional thermal overload does not provide short circuit protection. An alternative is a combination motor circuit breaker overload device, these have adjustment for correct setting of rated motor amps. Combination motor circuit breaker overload will protect the motor against excess current & also protect the motor leads / motor for short circuit - especially suitable if the motor leads are not rated for 500A.

14-03-2012, 11:46 AM
Thanks for your replys; hopefully I've attached a wiring diagram of what is currently installed; I propose to replace the contactors ( 6KM1,6KM2 , 6KM3 , 6KM4) with a Siemens sirus 3RW4076 soft starter ( 3 input, 3 output). You can see the installed feed is via 1QM1 & 1QM2 9 the 2 breakers.....250 amp) .
As previous I was hoping ( due to panel issues ) to keep the breakers 1QM1 & 2 & pair up the relevant phases from their respective L1, L2, L3 into the 3 inputs of the soft start.
FYI the compressor is a Bitzer CSH 9581 210Y ( 310 FLA). Was going to pair up cables (403, 409); (404,410);( 405, 411) which are all 95 mm2 . The currnet phase ocnductors 400,1,2,6,7,8 are copper bars ( unknown dimension ) so was going to replace them with something that was rated in excess of the circuit breaker value ( cant leave them in as they wont fit).

it was mentioned about the isolating of 1 circuit as opposed to the other but access to the electrics is via a panel door with interlocked isolator so didnt think that would be an issue.



All advise gratefully received.

14-03-2012, 07:53 PM
You will need to install one new circuit breaker supplying new soft starter

14-03-2012, 10:52 PM
With the new soft starter, and with the loadng/ current draw you have. It is common here to fit a parallel line contactor as well, pulled in after aperiod of time and motor full speed is acheived, saves nuisance tripping on occassions, interlocked through soft start of course.
Out of curiosity why are you changing the starter system. Looking at your schematics drg, it is a korndorfer standard auto trani /closed transition starter system [ old tech soft starter ] but with two supply breakers, a bit odd.