live to earth short circuit tripped mcb but not rcd?

[JohnW2]

Hello again, having done further inspection, it turns out if I touch the neutral and earth together the rcd does not trip at all, I tell a lie it does trip but only when heavy loads are switched on such as the kettle, ...

That is probably 'as expected' - touching neutral and earth will usually not make an RCD trip if there are no loads 'on' at all.

Kind Regards, John

 

[bernardgreen]

Do I have an earthing or insulation problem?

When there is a Neutral to Earth fault some of the Neutral current from a load will flow through the fault and bypass the RCD.

 

[jim63]

The thing is tho all circuits were on at the time, only small loads were being used however, just your usual fridge/freezer tv and a few lights ect, it used to trip when touching the neutral and earth when the central heating was running, it does not do this anymore which concerns me.

When there is a Neutral to Earth fault some of the Neutral current from a load will flow through the fault and bypass the RCD.
View attachment 150731

I appreciate how a rcd works but it is almost doing the complete opposite, Its almost like my rcd now needs a very high imbalance well in excess of 30ma to trip out, that also concerns me, I can now appreciate why the mcb had reacted faster but I still would have expected the rcd to kick in as a live to earth short is obviously still an imbalance. I can remember working on a property renovation which had wylex fuseboard and one of the workers damaged a cooker cable and this tripped both the mcb and rcd, you can see where my concern is.

Does my rcd need replacing like I say it is roughly 20 years and I will admit I have not tested it as regularly as I should have, the last thing I want is for someone to get killed because the rcd failed to operate due to possible age or damaged insulation in a cable somewhere, I suspect it may be a damaged neutral or earth maybe trapped by a socket screw or a wire has come loose somewhere.
thanks

 

[JohnW2]

The thing is tho there are loads on,only a very small amount, ie fridge tv ect, it used to trip when touching the neatral and earth when the heating pump was running, it does not do this anymore which concerns me.

Fair enough - if there has been a change, then that warrants investigation.

I appreciate how a rcd works but it is almost doing the complete opposite, Its almost my rcd now needs a very high imbalance well in excess of 30ma to trip out, that also concerns me,...

Possibly - but, depending upon how the circuyits are arranged, only a tiny proportion of the load current might go through your N-E connection - so it could still be 30mA or less even with fairly large loads...

Does my rcd need replacing like I say it is roughly 20 years and I will admit I have not tested it as regularly as I should have ...

What happens with the test button now. As far as we can make out, with most RCDs pressing that creates an imbalance of around 60mA.

Given the change in behaviour you describe, it sounds as if it would probably be worth getting the RCD properly tested (i.e. measure the trip threshold and disconnection speed).

Kind Regards, John

 

[jim63]

Fair enough - if there has been a change, then that warrants investigation.
Possibly - but, depending upon how the circuyits are arranged, only a tiny proportion of the load current might go through your N-E connection - so it could still be 30mA or less even with fairly large loads...
What happens with the test button now. As far as we can make out, with most RCDs pressing that creates an imbalance of around 60mA.

Given the change in behaviour you describe, it sounds as if it would probably be worth getting the RCD properly tested (i.e. measure the trip threshold and disconnection speed).

Kind Regards, John

Hi, the rcd trips when the button is pressed that has never changed, its more how it behaves towards the rest of the circuits it supplies if that makes sense, without spending fortunes on the proper testing equipment I cannot see a way to accurately test the rcd whether it will save your life or not without touching a live cable but I am obviously not willing to put that to the test.

 

[JohnW2]

Hi, the rcd trips when the button is pressed that has never changed, its more how it behaves towards the rest of the circuits it supplies if that makes sense ...

Well, as I said, if it still trips when the test button is pressed, it's trip threshjold cannot have increased to more than about 60mA.

... without spending fortunes on the proper testing equipment I cannot see a way to accurately test the rcd whether it will save your life or not without touching a live cable but I am obviously not willing to put that to the test.

As you say, not a recommended test! .. but, as you say, without the right test kit, there is no more you can do than you have. Replacing the RCD, if you are comfortable and competent to do it, would be much cheaper than buying test gear. An electrician could, of course, test the RCD in a couple of minutes, but I'm not sure what that would cost you.

Kind Regards, John

 

[Taylortwocities]

. Replacing the RCD, if you are comfortable and competent to do it, would be much cheaper than buying test gear. A

Yes, but a new RCD must be properly tested after replacement. An RCD tester is what is needed.

 

[JohnW2]

Yes, but a new RCD must be properly tested after replacement. An RCD tester is what is needed.

Yes, strictly speaking that's true. If replacing the RCD with a new one resulted in restoration of the behaviour (in response to the OP's 'shorting N-E' tests!) that would at least more-or-less confirm that the existing RCD was past its sell-by date but, as you say, the new one really ought to be tested.

Kind Regards, John

 

[SUNRAY]

So you have demonstrated the RCD works on the neutral (the description you have given is a common occurrence) so the Live side needs testing, this can be done by placing a small load between L & E. A crude way of doing this is to rewire a 13A plug for a small load, such as a table lamp.

AND THEN RESTORE THE PLUG IMMEDIATELY AFTER COMPLETING YOUR TEST.

 

[jim63]

So you have demonstrated the RCD works on the neutral (the description you have given is a common occurrence) so the Live side needs testing, this can be done by placing a small load between L & E. A crude way of doing this is to rewire a 13A plug for a small load, such as a table lamp.

AND THEN RESTORE THE PLUG IMMEDIATELY AFTER COMPLETING YOUR TEST.

Never thought about that, sounds like a handy trick and very simple, I will give that a go,its certainly sounds much safer than grabbing hold of a live wire me thinks

 

[Echo the husky]

its certainly sounds much safer than grabbing hold of a live wire me thinks

Not as much fun though.

 

[JohnW2]

So you have demonstrated the RCD works on the neutral (the description you have given is a common occurrence) so the Live side needs testing, this can be done by ....

Since an RCD detects the L-N current difference, it's hard to see how it could behave differently with L-E and N-E 'faults'.

However, I suppose what you suggest could be done to roughly quantify the RCD's trip threshold - assuming that the OP's supply voltage is around 240V (which is what most of us get), 30mA would correspond to roughly a 7.2W (resistive) load - so an RCD ought to trip with a (restive) L-E load of about 7.2W or more, and should not trip with an L-E load less than about 3.6W. Assuming he hasn't got a box of low-value wirewound resistors, he might struggle to find resistive loads that small - unless he has a stockpile of very low wattage incandescent bulbs.

Kind Regards, John

 

[jim63]

I have just tried that trick on my polaroid 32 led tv, I connected the neutral wire in plug to the earth terminal then switched on and to my surprise the rcd did trip, the tv draws about 60w max when running in use, it was on standby so I am not sure how many watts it drew, its just after midnight, so hardly anything is being used at the moment, but somehow this experiment managed to trip the rcd, so maybe it does work after all and would save my life. Thank god i can get some sleep now.

 

[Echo the husky]

There was a bad batch of MK RCBOs a while back that would trip at their rated tripping current but would not trip when tested at 5x their tripping current.

 

[SimonH2]

I would suggest it could actually help in that (like in many other places) it can now be suggested to test when the clocks change (as they will need reset anyway).

Well I suppose that helps. In many cases there's a distinct reluctance to test the RCD(s) regularly because of the complete PITA of resetting all those bits of kit where the designers couldn't be ar*ed to provide even a few seconds worth of state retention. My old alarm clock was a particular PITA to set - but it died a few months ago after several decades of use. The new one (combined clock & DAB radio) is great - it remembers it's settings and sets the time itself
We've a music centre downstairs that doesn't even remember the radio presets after anything but the shortest of power interruptions

I was doing some work yesterday that stood a good chance of tripping the RCD that's in the feed to my servers - so I took this as a hint to get around to moving them to the UPS that now has working batteries in it. I managed not to trip the RCD though.