wireing

[racer2]

Hi, I triped the correct MCBs befor changing the sockets in my house which was fine, but when ever the cables touched one another the main MCB tripped, is this right?
Also when i turn the hoover on the lights dim for a millisecond, not a problem but curious as to why this should happen.
Thanks for any replies

 

[sm1thson]

Hi, I triped the correct MCBs befor changing the sockets in my house which was fine, but when ever the cables touched one another the main MCB tripped, is this right?

if the 'main MCB' isnt an MCB but an RCD (if it is it will have a test button) then yes its fine, what is happening is when you short earth and neutral a small amount of the neutral current (from the other live circuits) takes its path through your cable and to earth thus causing it to trip.

if it was an MCB then something is very wrong. but there would have been a flash and a bang so you would know that already.

Also when i turn the hoover on the lights dim for a millisecond, not a problem but curious as to why this should happen.
Thanks for any replies

yeah its (generally) normal. V=IR. all cables and joints have a resistance (R) when you start a motor it uses a lot of current (I) so I*R=largerV this V is the voltage dropped across the cables and joints up to where the lighting circuit splits. as more voltage is dropped across the cables there is less across the lights thus they are dimmer.

if there is a loose connection in/before your consumer unit it will do it more, but its normal to do it to an extent.

when I was last in a Hotel in the USA you should have seen how much the lights dimmed when i plugged the iron in! -but there there is less voltage to start with, more current needed to compensate (so more drop), also the lights and iron were on the same circuit.

 

[stem]

What probably happened is that you switched off the MCB for the individual circuit and allowed the neutral to touch the earth which tripped the RCD (the device with with test button). If so that's normal.

MCB's prevent overload of the circuits
RCD's detect a fault to earth

Most peoples vacuum cleaners & kettles etc dip the lights marginally due to a start up surge, again a small dip is quite normal.

Edit - Beaten by seconds & Sm1thsons reply's better

 

[Steve]

I was in a large new house in Canada, relatives house, their washing machine used to cause the hall light to go almost completely out for a minute at a time, I guess while the heater was on in the WM. I'm guessing this house was some distance from the nearest substation, it was part of a large residential estate of top spec houses, perhaps the supply was undersized.

My fridge freezer draws a large amount of current on startup - you can almost hear it pulling the current as it starts. Makes a lot of noise for the first few seconds until its calmed down.

 

[racer2]

Thank you, yes it was the RCD