MCB Loud buzzing noise

[Dazza1976]

Hi all, can anyone help please?

I installed 18 downlights today, in 2 rooms with 9 in each room. and ran them on a completely new circuit with a 6amp breaker. When turning on the lights there is a very loud buzzing sound coming from the mcb ??? Both switches are dimmers. I thought perhaps the mcb is too close to overload ?? when i tried replacing the dimmers with standard one way switches the buzzing stopped ??
Can anyone help please as the customer is adament they want dimmer switches, and im not happy leaving the job with the loud buzzing.


Thanks in advance.

 

[pushrod]

Can anyone explain why this is ?? as mentioned by someone else 18 x 50w = 3.91A, so its not really as if a 6amp breaker was being overloaded ??

It is overloaded because of the fact that you are dealing with an inductive load. If you only have wattage ratings to work with - which is the true power - you actually have to multiply it by 1.8 to get the apparent power which is what you have to size your cable for.

Quite a common problem - this is why the c&g 2330 301 exam constantly bangs on about kW, kVA and kVAr. Hope that is a bit clearer.

 

[markyyy]

the consumer unit and mcbs are crabtree. i will probably need to look further into this as it is an annoying buzz. i could use one transformer for two fittings with 20w lamps, but that again is more time on the job.

 

[pushrod]

the consumer unit and mcbs are crabtree. i will probably need to look further into this as it is an annoying buzz. i could use one transformer for two fittings with 20w lamps, but that again is more time on the job.

did you try decreasing the load or increasing the breaker size?

 

[Crosswire]

It is overloaded because of the fact that you are dealing with an inductive load. If you only have wattage ratings to work with - which is the true power - you actually have to multiply it by 1.8 to get the apparent power which is what you have to size your cable for.

Quite a common problem - this is why the c&g 2330 301 exam constantly bangs on about kW, kVA and kVAr. Hope that is a bit clearer.

Hmmm , well spotted but OP said "when i tried replacing the dimmers with standard one way switches the buzzing stopped ??"

If the problem was a (slightly) overloaded breaker, why should removing the dimmer from the circuit, also stop the buzzing?

Not trying to contradict you, just curious.

 

[Crosswire]

Unless dimmer switch also acted as an inductive load in itself.....Is this possible? Anyone?


Just thinking out load, please don't gun me down if I'm being a fool

 

[pushrod]

Unless dimmer switch also acted as an inductive load in itself.....Is this possible? Anyone?


Just thinking out load, please don't gun me down if I'm being a fool

Dimming is quite a difficult subject which i only know a little about. The very old dimmer switches which were basically a coiled variable resistor/potentiometer (were very wasteful of energy)so would have been inductive.

Modern dimmers are more efficent and use electronics in different ways, some can use inductive phase shifting via inductive elements in the circuit and others operate by using thyristors for high speed switching of the wave form to reduce the average voltage applied. These ones however, i believe, also then use inductors to suppress radio interference.

So yes it is likely that the dimmers themselves are inductive.


edit, btw loved the "thinking out load"