Gentlemen

I am currently having problems with our main LV Incomer

Merlin Gerin
Masterpact H1
1600A
STR38S

We are having nuisance tripping with this unit

We are 95% confident there are no physical faults.

We believe the problem is the breaker itself.
It seems to work better with more load going through it for some reason !

Any help is greatly appreciated.

Kind Regards
simon

These Merlin Gerin ACB trips have a history of 'tripping' when operating with a load below 20% of the nameplate rating. The relay in the ACB is 'self powered' via the load passing through the ACB... if you have a load below 20% the trip is effectively 'off' until you have a fault and then it comes back 'on'.

The 'low load' issue has been a problem for many years with no immediate fix.... MG will not admit that the problem is with their ACB STR trip design.... An auxiliary power supply can be added to 'hold' the ACB up under low load conditions, (it was actually designed to be utilised for metering under low loads) however, this is not guaranteed.

The options are:
1. Have the trip calibrated to see if you have a faulty one and replace.
2. Replace the trip with an external relay with DC battery power or a self powered external relay. Available from Schneider under the MiCOM brand.....
3. Fit the external power supply to the ACB and hope for the best...

Aside from the obvious like short time current pick up settings I'd go along with MickyB's comments on low load through the unit - I've had the same problem myself.

How old is the ACB - and has it ever been subject to injection testing - if not that might be one approach (but presumes you have a spare to use (or can beg, borrow or steal one)

There are some (unconfirmed) reports that the CT's within the breaker have been reversed at assembly which seems to exacerbate the problem with "unidentified tripping"

Regards

OMS

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Failure is always an option

To MickeyB and OMS thanks very much for that info that is very interesting, as we do have over half of our plant off and it is below the 20%, we now have something to go on, thanks sime :-)

Hi
you have another option ,there are replacement trip units available and these options are tried and tested with over 600 Merlin Gerin STR units being replaced by major players in the telecoms industry.
I found these on masterpactcenterofexcellence.co.uk

sil9090

I am interested to know if you ever have found out solution for the nuisance tripping you have had. I also have similar tripping problem with the STR38 S tripping unit I have.

Thank you very much.

BESAT

Hi
yes we changed out the faulty units with an off the shelf alternative from p s electrical services ltd ,they are based in Kidderminster on Ikon Trading estate ,Droitwich Road Kidderminster DY10 4EU
Regards
sil9090

Hi Sil9090;
You mean you replaced just the trip units, and not the whole breaker. I try to search the adress you mentioned. Thank you for the update.

Besat