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

[PatH]

Hi
I have a problem on a site that I have inherited.
The fault is that a 100A MCCB is tripping indeterminately.
The MCCB feeds a DB which has several cct’s none any higher than 32A and all cct’s are RCBO’s the RCBO’s are not tripping yet the MCCB is.
The MCCB has been changed recently.
Ahead of connecting up a Power Quality Digital Storage Oscilloscope which incidentally I haven’t got.
Any ideas?

PatH

 

[Marvo]

Check that it's not a poorly fitted lug or termination on the MCCB getting hot when under load.

 

[davesparks]

Is it overloaded?

 

[Simlec]

Is it set correctly.

 

[davesparks]

It can be set correctly yet still be overloaded

 

[Simlec]

It can be set correctly yet still be overloaded

Then it's doing what it's made for.

 

[davesparks]

Then it's doing what it's made for.

Sorry, missread your post as 'it is' rather than 'is it'

 

[Simlec]

Where is PatH?
is it single pole or 3/4 pole ?
what type is the outgoing cable?

 

[Engineer54]

I'm going to assume this is a 3 phase MCCB, if so have you checked the load balance (eg, is there one phase grossly higher loaded than the other two phases, does it have adjustable parameters, if so are the settings correct? Does it have earth fault protection, again is the setting(s) correct....


If all the above is fine then take regular amp clamp readings throughout the day, or hire a power analyser, because it would seem that on at least one phase the MCCB is seeing an overload....

 

[PatH]

I'm going to assume this is a 3 phase MCCB, if so have you checked the load balance (eg, is there one phase grossly higher loaded than the other two phases, does it have adjustable parameters, if so are the settings correct? Does it have earth fault protection, again is the setting(s) correct....


If all the above is fine then take regular amp clamp readings throughout the day, or hire a power analyser, because it would seem that on at least one phase the MCCB is seeing an overload....

Hi Guys

Thanks for your comments
I have tried most everything that you have mentioned
Loading on the 3 phases equal
Terminations are tight
The MCCB is not adjustable
The temperature of the cables are not excessive
The MCCB is the magnetic type so it is susceptible to a current crest factor 141.14amps on this breaker.
I have a suspicion that it may be this that is the problem. 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) 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.

PatH

 

[Deleted member 72666]

Hi Guys

Thanks for your comments
I have tried most everything that you have mentioned
Loading on the 3 phases equal
Terminations are tight
The MCCB is not adjustable
The temperature of the cables are not excessive
The MCCB is the magnetic type so it is susceptible to a current crest factor 141.14amps on this breaker.
I have a suspicion that it may be this that is the problem. 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) 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.

PatH

Did you just make that bit up?

 

[PatH]

Did you just make that bit up?

LOL what bit Stan

 

[PatH]

H?as anyone had any similar problems

 

[shanky887614]

LOL what bit Stan

www.ccontrolsys.com/w/Current_Crest_Factor

i hadn't heard of it either, it makes sence considering ive looked into rms briefly

 

[Marvo]

Unless your peak load current is very close to the MCCB's 100A or unless there's large motor drives on the load I doubt that crest factor is the problem.