RCD didnt trip!

[oldfart]

Hi all
Please help, got a nice new split load rcd protected cu.

Over the weekend my wife was using the dreaded hedge trimmers and cut through the cable, bit of a spark but she was ok, my main concern is that the rcd side of the board didnt cut the juice! all the sockets are on the protected side cant understand why.

I,ve checked the test facility and it cuts off when pressed.

The hedge trimmers were double insulated and therefore no earth present, could this be the reason?.

I,m suprised a major fault like that did,nt kill the juice, I would welcome any comments

thanks

 

[Spark123]

The explination is most likely there was no path for the current to flow to earth, i.e. your wife didn't get a shock or the exposed conductor didn't touch earth. The spark you saw was probably the live and neutral conductors shorting together by the hedge trimmer, possibly popping the plugtop fuse?

 

[Adam_151]

An rcd detects inbalence between live and neutral, which would be caused by current taking an alternative path, either through a person, or through the case of a class 1 appliance and through the cpc.

The appliance with a damaged flex probably was class 2, meaning it did not need an earth connection and so the cable didn't have an earth core, so when the cable was cut, thats not a fault the rcd can protect against, however the over current protection might open, the reason we use rcds, is because this can't be garenteed (if the blade that cut the cable was painted, rusty, etc, countless reasons), the rcd is to protect you if you are foolish enough to pick up the cut cable without switching off first, if you touch the exposed conductor (direct contact), power would flow through you to the ground, this has the possibility of being fatal, and overcurrent protection won't make it any less so, this will however create an inbalence which will open the rcd

 

[Darkness]

I thought that when the RCD test button was pressed, a small current was passed from the live to the neutral causing the RCD to trip?

So when the live and neutral touched in the hedge trimmer, I thought that would have tripped the RCD?

 

[Spark123]

I thought that when the RCD test button was pressed, a small current was passed from the live to the neutral causing the RCD to trip?

When the test button is pressed on an RCD, a trip current is passed from the Live on one side of the RCD (i.e. L in) to the Neutral on the other side (i.e. N out), causing an imbalance and it to trip.
http://www.tlc-direct.co.uk/Book/5.9.2.htm

So when the live and neutral touched in the hedge trimmer, I thought that would have tripped the RCD?

Not necessarily, an RCD is designed to trip on the imbalance of the live and neutral currents flowing through it. If there is no earth path for a fault current to flow, the RCD will still be in balance and will not trip.

 

[oldfart]

Thanks guys for your detailed replies I now understand. I,ll let her use the lawnmower next !

 

[andy]

Thanks guys for your detailed replies I now understand. I,ll let her use the lawnmower next !

is it cordless?!

 

[Darkness]

Why don't they have an RCD test button also to allow a current to flow to earth to test whether your earth is ok?

It could prove to be quite helpful.

Or is it the fact that it would be dangerous to allow a current to flow to earth i.e. someone touching a metal item that has an earth connection?

Suppose the con's massively outway the pro's for it.

 

[baldelectrician]

Why don't they have an RCD test button also to allow a current to flow to earth to test whether your earth is ok?
quote]

that's why test equipment incorporates a rcd tester- to make sure a rcd is tripping within tolerances remote from the actual rcd

 

[andy]

Why don't they have an RCD test button also to allow a current to flow to earth to test whether your earth is ok?

It could prove to be quite helpful.

Or is it the fact that it would be dangerous to allow a current to flow to earth i.e. someone touching a metal item that has an earth connection?

Suppose the con's massively outway the pro's for it.

just because there is an earth upto the RCD doesnt mean there is an earth in the lead to the appliance. if she had of touched the cable once chopped (providing it was still live) then she would have got a shock, tripping the RCD

 

[Darkness]

Cheers for that folk.

 

[ban-all-sheds]

The appliance with a damaged flex probably was class 2, meaning it did not need an earth connection and so the cable didn't have an earth core

With garden appliances like that I think there is a lot to be said for still using 3-core flex, with the cpc connected at the plug but not at the appliance end.

That way, when an incident like this happens, the RCD will trip, possibly before you've blown a chunk out of the blade of whatever chopped through the cable.

 

[ebee]

The appliance with a damaged flex probably was class 2, meaning it did not need an earth connection and so the cable didn't have an earth core

With garden appliances like that I think there is a lot to be said for still using 3-core flex, with the cpc connected at the plug but not at the appliance end.

That way, when an incident like this happens, the RCD will trip, possibly before you've blown a chunk out of the blade of whatever chopped through the cable.

A very good point