100 A MCCB feeding DB with various RCBO's tripping before they do?

Engineer54 · Sep 18, 2014

PatH said:
Hi Guys

Thanks for your commentsI have tried most everything that you have mentioned
Loading on the 3 phases equal Terminations are tightThe MCCB is not adjustable
The temperature of the cables are not excessive What about the ambient temp of it's enclosure?
The MCCB is the magnetic type so it is susceptible to a current crest factor 141.14amps on this breaker. All standard MCCB's have both thermal and magnetic element trip release's
Ihave a suspicion that it may be this that is the problem. I doubt that assumtion I also have the suspicion that the MCCB trips on start up in the morning. (This is very hard to judge as it does not happen every day) Wouldn't the store manager know if the DB is tripping out first thing in the morning One of the cct’t from the DB feeds a gondola with a number of phones, chargers and associated electronic equipment. I did ask the store to leave this cct off for a fortnight to see if the problem went but they had various visitors in which they could not carry out this request. I have tested this cct for Zs and IR and all is fine also the RCCBo does not trip on this cct. I think I probably will have to clamp the phases as the store starts up in the morning and turns everything on.

Any Air Conditioning in this store by any chance?? If so is it a central unit or more than one smaller split AC units?
What's the max current draw on the 3 phases that you have measured?

Why not get this store to invest in a weeks hire of a power analyser, a decent analyser will a clear indication of what's going on within several parameters, not just current draw.... The forum sponser hires such analysers, give them a ring for a quote

davesparks · Sep 18, 2014

The last time I hired one it was only £200 for the week including delivery, but that wasn't from the sponsor so I dare not mention the name of the company.

PatH · Sep 19, 2014

Engineer54 said:
Any Air Conditioning in this store by any chance?? If so is it a central unit or more than one smaller split AC units?
What's the max current draw on the 3 phases that you have measured?

Why not get this store to invest in a weeks hire of a power analyser, a decent analyser will a clear indication of what's going on within several parameters, not just current draw.... The forum sponser hires such analysers, give them a ring for a quote

Hi Guys

Thanks for your comments it gives me a little more to work with. The store manager has called our engineer out at various times of the day and to be fair I am not sure that they pay that much attention to the gondolas until someone points it out to them. I have asked them to monitor this but there are more than one shift manager and they rotate their duties. They are more concerned with sales etc so that is the situation with this problem. I have only visited site twice. (I tend to inherit the problems nobody else can figure out) And I have simulated start up on one occasion but the MCCB did not trip. I am trying to figure out the common denominator and what makes the conditions just right. There are several split ac systems on site but I am sure they are on their own dedicated DB (I must verify this) and fed from another MCCB. I may not be back on site for a couple of weeks due to other commitments but I will post with what I find (for anyone that's curious).

PatH