Nuisance tripping

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What is the correct way to install an outside rcd socket ?

1). Connect to ring main, but when test button is pressed it takes out whole of the house because both main and socket are 30ma rated.

2) Replace main RCD in consumer unit with a higher rated tripping current.

3) Use an ordinary outside weather proofed socket.

4). Avoid pressing the test button altogether ( but someone else might when
we're not there) away on holiday for example) and come home to all the
problems of de-frosted freezers etc.

Ps. If (3) is an option where do you purchase as most Rcds are 30ma rated
 
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3 - if your socket circuit is RCD protected the outside socket does not require separate RCD protection.

http://www.screwfix.com/p/bg-nexus-13a-2g-dp-switched-socket/67928

Most would recommend connecting the outdoor socket to your 'ring main' via a dual pole isolator. This would allow you to isolate the outdoor socket should any faults occur.

You could use this with a 13 amp fuse although the total load on the socket would be 13 amps:
http://www.screwfix.com/p/british-general-13a-double-pole-switched-fused-connection-unit-white/86116
 
1). Connect to ring main, but when test button is pressed it takes out whole of the house because both main and socket are 30ma rated.

This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question.

If pressing the test button does trip other units then something else is wrong!
 
1). Connect to ring main, but when test button is pressed it takes out whole of the house because both main and socket are 30ma rated.

This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question.

If pressing the test button does trip other units then something else is wrong!

How is that configured if no current flows to ground. What goes in must come out (somewhere)
 
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1). Connect to ring main, but when test button is pressed it takes out whole of the house because both main and socket are 30ma rated.

This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question.

If pressing the test button does trip other units then something else is wrong!

How is that configured if no current flows to ground. What goes in must come out (somewhere)


Yes & no!

An RCD works by sensing the difference between the current flowing 'in' on the phase conductor & 'out' on the neutral conductor. It does not sense current on the earth. For a healthy circuit the current on the neutral conductor is precisely the same as the current on the phase conductor. If you subtract one from the other the answer will be zero. This is what the RCD does.

The test resistor inside an RCD is arranged such that it is connected to the phase conductor after the measuring coil but to the neutral before the measuring coil. Hence the test current is 'seen' by the phase measure but not by the neutral measure. This creates an imbalance & the RCD trips. No current is passed to earth.
 
This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question. If pressing the test button does trip other units then something else is wrong!
We've discussed this before. Whilst what you say is certainly true of passive RCDs in CUs and standalone RCDs, it appears that the test buttons of at least some RCD sockets (and maybe RCD FCUs) do (for whatever reason!) create a L-E 'leak' (i.e. the resistor goes from the L output of RCD to earth) - and so can cause any other RCDs on the circuit to trip.

Kind Regards, John
 
This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question. If pressing the test button does trip other units then something else is wrong!
We've discussed this before. Whilst what you say is certainly true of passive RCDs in CUs and standalone RCDs, it appears that the test buttons of at least some RCD sockets (and maybe RCD FCUs) do create a L-E leak (i.e. the resistor goes from the L output of RCD to earth) - and so can cause any other RCDs on the circuit to trip.

Kind Regards, John
I have first hand experience of this a 10 mA RCD socket on our test bench would take out the 100 mA RCD on our main box. But standard ramp test with RCD tester would not.

The 10 mA RCD are hard to find and often rather expensive. Since already RCD protected a simple FCU so you can isolate both live feeds (line and neutral) is likely best option.

Or super glue the test button so can't be pressed. That is what we did and since we had a plug in tester there was in our case no need for the inbuilt test button.
 
This should not happen. The test button on a RCD creates an imbalance between the L & N by applying a resistor across the coil inside the unit. It doesn't work by passing a current to earth. Pressing the test button is invisible to any RCD upstream (or down stream for that matter) of the RCD in question. If pressing the test button does trip other units then something else is wrong!
We've discussed this before. Whilst what you say is certainly true of passive RCDs in CUs and standalone RCDs, it appears that the test buttons of at least some RCD sockets (and maybe RCD FCUs) do (for whatever reason!) create a L-E 'leak' (i.e. the resistor goes from the L output of RCD to earth) - and so can cause any other RCDs on the circuit to trip.

Kind Regards, John

This was my experience. The SafetySure range of sockets from Screwfix (and other places) seem to cause a L-E 'leak' however, the Powerbreaker FCU I have upstairs seems to use the L-N method as it never trips my main RCD.

It was because of this, and advice on this forum, that I replaced the SafetySure RCD sockets with standard metal clad sockets. The RCD test button was the only way to switch off power to the appliance (there was no seperate on/off switch).
 
This was my experience. The SafetySure range of sockets from Screwfix (and other places) seem to cause a L-E 'leak' however, the Powerbreaker FCU I have upstairs seems to use the L-N method as it never trips my main RCD.
When it came to my attention that some of these RCD sockets (and maybe some RCD FCUs) used a leak to earth (rather than the 'L-N method') for testing, I started wondering why on earth they would do that.

The only thing I could think of was that one advantage of the test button introducing an L-E leak is that it also tests for the presence of an adequate connection to earth (adequate enough for RCD operation), as well as testing the actual RCD operation. However, goodness knows whether that was their thinking!

Kind Regards, John
 
As you say, it could be seen to be a better test than a "leak-free" imbalance.

And as it's reasonable to think that you wouldn't have an RCD socket on a circuit which already had an RCD....
 
As you say, it could be seen to be a better test than a "leak-free" imbalance.
Yes, it certainly tests more than just the RCD. A problem, however, is obviously that the user/house owner/DIYer might, as a result of the 'failed test', replace a perfectly good RCD socket (and not cure the problem) if the actual problem was an inadequate earth.
And as it's reasonable to think that you wouldn't have an RCD socket on a circuit which already had an RCD....
It might be reasonable to think that, but it's not unusual for us to hear of situations in which such is not the case.

Kind Regards, John
 
…and - could it be because RCCBs have no connection to/for earth? Too simple ?
Well, that's obviously looking at it the other way around - that the manufacturers of all residual current devices would like the test button to test both the adequacy of the earth and the functioning of the device, but they were not able to do that with devices (like standard RCDs) which have no earth connection.

I suppose that is a possible explanation but, if they felt strongly that such test functionality was desirable, they could presumably have made provision for an earth (like the 'functional earth' lead of RCBOs).

Kind Regards, John
 

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