RCD Trouble

americanpie

Testing RCD in a board today, old installation, without a tester, just shorting the earth and neutral in a plug top and plugging it into each socket in the house. Didnt trip at all.

Theres a supply to an outside office looped from the board to it's own board and that RCD wouldn't trip either.

There was no earth rod installed so I put one in and it helped. The RCD in the office worked perfectly after this.

But the one in the house, tripped sometimes and other times it didn't, in the same sockets.
Shorted it at the board as well, somtimes trip and sometimes didnt.

So thought faulty RCD got another one and same thing happened, any ideas or did I just get 2 faulty RCDs?

Bruthal

americanpie wrote: »

Testing RCD in a board today, old installation, without a tester, just shorting the earth and neutral in a plug top and plugging it into each socket in the house. Didnt trip at all.

Theres a supply to an outside office looped from the board to it's own board and that RCD wouldn't trip either.

There was no earth rod installed so I put one in and it helped. The RCD in the office worked perfectly after this.

But the one in the house, tripped sometimes and other times it didn't, in the same sockets.
Shorted it at the board as well, somtimes trip and sometimes didnt.

So thought faulty RCD got another one and same thing happened, any ideas or did I just get 2 faulty RCDs?

The neutral to earth thing will usually only trip the RCD if there is a load on the circuit your shorting the neutral to earth on. If there is no load and so no current flow in the circuit then there will be no neutral current to divert to the earth in the plug you rigged up. Very small currents can divert from other circuits to the earth in that situation but usually so small it wont trip the RCD, so it needs a load on the circuit your plugging into.

M cebee

the rod doesn't matter when testing rcd -assuming the installaion is 'neutralized'

if it's an old installation with TT you'd need to ensure ADS (main rcd/rod)
or rewire/upgrade to tn-c-s if allowed

Bruthal

M cebee wrote: »

the rod doesn't matter when testing rcd -assuming the installaion is 'neutralized'

if it's an old installation with TT you'd need to ensure ADS (main rcd/rod)
or rewire/upgrade to tn-c-s if allowed[/QUOTE


yea the earth rod newly installed in office probably made no difference, was probably just a load on the circuit the next time he tried the neutral earth bridge plug. It usually wont trip doing that when there is no load on the circuit.

But after all, it was not a proper RCD test done here which takes a small measured current from Live to Earth in a proper RCD tester.

M cebee

does the fault loop impedance affect the rcd tester at all?

say if you had 'TT' with loop impedance(Zl) of 200ohm or more

i know it doesnt affect the test button

Bruthal

M cebee wrote: »

does the fault loop impedance affect the rcd tester at all?

say if you had 'TT' with loop impedance(Zl) of 200ohm or more

i know it doesnt affect the test button

The RCD tester applies a small current from live to earth, its an interesting question you ask, because if the loop impedence was as high as 200 ohms then the RCD tester would have to apply a higher voltage between live and earth to get the 30ma test level. Im not sure if the RCD tester does that, as in measures the earth impedence and applies the voltage needed to get the setting the user has picked, or its just assumed the earth loop impedence will be close to zero. That would be an interesting test to try.

As for the test button, no current goes down the earth conductor when RCD test button is pressed, it just bypasses one of the current coils and so trips it that way, so the test button would work even on an isolated non earthed setup.

M cebee

think it works ok -meter compensates i guess

after all they use the same meters in uk, where tt is common


pressing the 'test buton' is the way to test anyhow in the absence of 'rcd tester'

Bruthal

M cebee wrote: »

think it works ok -meter compensates i guess

after all they use the same meters in uk, where tt is common


pressing the 'test buton' is the way to test anyhow in the absence of 'rcd tester'

It is, but the tester does confirm there is at least some earth path, as does the neutral earth short at a socket, but the neutral earth thing definitely requires a load to be on. Using an ammeter between neutral earth will show the current that flowed to trip RCD but would need one if them peak recording readings on it as it will trip instantly if its going to.

Good point about the uk tt and the rcd testers working there, so that looks like they do bring the current up to the user set level unless there is no earth at all.

M cebee

shorting neutral-earth at plugs is totally 'off the wall'

you could have reverse polarity at a plug-

and cause a high 'touch voltage' without the ADS(main rcd) present if it's an old TT installation

Bruthal

M cebee wrote: »

shorting neutral-earth at plugs is totally 'off the wall'

you could have reverse polarity at a plug-

and cause a high 'touch voltage' without the ADS(main rcd) present if it's an old TT installation

Well i was`t advocating it, but its not that serious, ye check socket voltage/polarity first if doing that, I have actually done it before myself, but its not a proper RCD test, but it is a decent indicator to show its working. I dont see a major problem with it myself. Its hardly a major ordeal to verify socket polarity. And if it was reversed and it was old tt system as mentioned above and failed to trip RCD then its better to see it there than a fault occuring and going un-noticed.
But obviously the propper tester is the ideal method.

A 10 watt bulb would be a good tester, wired to a plug between live and earth. That draws about 40ma`s

Randyleprechaun

Get the installation checked out by a qualified electrician who has a proper tester and knows how to use it.

Pressing the test button on an RCD only verifies that the RCD trip mechanism is mechanically ok. It's not meant to verify that it is working electrically.

Bruthal

Randyleprechaun wrote: »

Get the installation checked out by a qualified electrician who has a proper tester and knows how to use it.

Pressing the test button on an RCD only verifies that the RCD trip mechanism is mechanically ok. It's not meant to verify that it is working electrically.

Id have to partly differ with that, the test button does an electrical test on the RCD. It uses an electrical differential to test the mechanics of its operation. If the test button does not operate it then a new RCD is needed no matter what a tester says.

2011

Pressing the test button on an RCD only verifies that the RCD trip mechanism is mechanically ok. It's not meant to verify that it is working electrically.

Incorrect. I would have to agree with Robbie here.

First: If this was a mechanical test the RCD would operate when disconnected.

Secondly: Study the diagram below. The test switch works just as Robbie explained above.

Randyleprechaun

GRANTED!!

But it still is a worthless test as it doesn't give any indication whether the RCD will trip under fault or not. Basically all it tells you is that the RCD is capable of tripping.

However it doesn't say that it will or in the specified time range.

2011

But it still is a worthless test

No. Many people dont own RCD testers, yet they have found out that their RCD is faulty thanks to the test button.

it doesn't give any indication whether the RCD will trip under fault or not

It shows that the RCD has the potential to operate under some fault conditions. Not perfect, but better than nothing! From my experience RCDs generally work within the speciified range or they dont work at all.

Basically all it tells you is that the RCD is capable of tripping.

Yes.

However it doesn't say that it will or in the specified time range.

Agreed.