Re: Circuit Breaker

Spir Georges GHALI

nathan said:

Hi. sir me again.i just wanna ask about CB rating 1250 Amperes 3 pole. there is a humming sound and i touch the CB and the wire in a line side its very hot. the wire size is 500 AWG .3 sets of 500 AWG.what will be the cause why there is a humming sound? why the wire is so hot? i am afraid that will be a cause of burning or the CB will tripped off.what will i do. 

 i hope sir you will answer my question. Thank you very much. God bless.



Laurent said:

I'm not familiar with AWG cable sizes, difficult for me to tell if sizing is ok or not.

Did these noises and over-heating appear only recently? If yes, did anything change in your installation recently?

Some extra questions:

– Do you know the actual (rms) current going through this breaker? What kind of loads are fed by this CB ?

– Do you have the possibility to make an infra-red measurement, to show the temperature values around this CB ?

– Is there any relation between this CB and the transformer you mention in your other topic, which also makes noise? Is this CB fed by this transformer? How are they connected together, cable or busway?

What I'm thinking about is that you might have a high level of harmonics in your installation, which could be a reason for vibrations and over-heating. Or maybe a connection getting loose…

Dear Mr. Nathan ;


All reasons were been defined by Mr. Laurent are possible to have a hot circuit breaker, but refer to my experience, to say about CB is hot is not enough, because you should measure the temperature of the circuit breaker that means : the temperature of connection points and the temperature of the body, them we can decide if the CB is hot more than standard or not. noting that the ” IEC 947-1  &  IEC 947-2 ” said the the heating of connection points above the ambiant temperature should ” maximum + 60°C “, and for the touchable body ” maximum + 50°C “, with one condition that the maximum value of the ambiant tenperature is ” maximum + 40°C “.


You replyed to Mr. Laurent that the sention of cables are ” 3 x 250 mm² “, and to say it's enough or not we should know the working conditions that means : the kind of cables, the methods of installation, the ambiant temperature, and if there's another circuits on the same way, but also from my experience i think it's not enough. Noting that if the heating of cables can affect the conection points of CB


You also replyed to Mr. Laurent that the measured temperature is ” + 40°C “, if it's realy like that the CB is not hot, or you mean that's the ambiant temperature ? Please re-check again.

Rem. : I measured the temperature of ” CB 1600A ” with ” 1450A ” as the total load, in ambiant temperature ” + 30°C ” the temperature of connection points were ” + 84°C “, and regarding the standard values it's OK.