Paul,

I would suspect that the upstream primary device has a time delay setting equal to or less than the final device at the caravan connection, 


Cheers GTB

If you have two non delayed RCDs cascading it could be one or the other or both that trip


If one is delayed then it should trip last or not all , however you really need to densitive by a factor of three to be sure i;e:-

10mA standard & 30mA delayed.

30mA standard and 100mA delayed.

for example


If you test the upstream one on its own and get readings then there is a little trick you can do to bypass it to test the downstream one. But you need to know what you are doing otherwise potentially dangerous

It's a case of fastest finger first. They both see the same fault at the same time and the fastest wins, of course they sometimes both trip. The standing load on the main RCD may give it a head start as I notice that when there are two RCDs in series if only one trips it is usually the furthest away but I have always put that down to sods law but I could be wrong. I am sure there is someone on here who can give a more technical explanation  ?

To test a downstream RCD without firing off the upstream one, use the neutral of the upstream supply as the 'earth' for the tester. Having first verified supply polarity ! The RCD under test sees the imbalance when the button is pressed, as current flows out of the RCD live and does not come back via the neutral, the upstream RCD sees equal and opposite L-N currents and does not trip.


It may be the non-tripping RCD is faulty, or it may be that the two RCDs are a too close in rating, and one is nearer the centre of permitted performance, and the other nearer the edge. If they have identical spec, say both 30mA instant, then  which goes first will always be a toss up.

Or it may be that the upstream RCD is already taken near to the brink of tripping by other loads and needs less to take  it over the edge.

What are the two RCD types and ratings, and what is the level of  standing leakage ?

BS 7671 in general requires the settings of RCDs to be coordinates so that downstream RCDs are unlikely to trip.


HOWEVER, that are certain cases where this is not possible.


For example, in this case, BS 7671 has a safety requirement for all socket-outlets to be protected by a 30 mA non time-delayed RCD, as well every circuit in the caravan that's plugged into those socket-outlets.


Because the devices have the same rating (30 mA, non time-delayed) to meet BS 7671 requirements, either RCD might have a faster response, so either may trip before the other.


In these situations, the only way to test the downstream RCD (the one in the caravan) properly, is to use what is known as the "up/down" or "upstream/downstream" method ... IF that is supported by the test equipment manufacturer.

 
Of course, this isn't as simple as "plug in and hit the test button", and requires the devices under test meet IP2X/IPXXD with covers removed, and test equipment and test leads to comply with HSE Guidance Note GS38.

"To test a downstream RCD without firing off the upstream one, use the neutral of the upstream supply as the 'earth' "

tst tst I wasn`t going to mention that!     Although I have been known to do it meeself

Yes, the method uses the Neutral to test the RCD.


Is that an issue?

1. Where the RCD is used for additional protection, as per the requirement in 411.3.3 or 708.415 for the socket-outlet supplying the caravan and 721.415.1 for the caravan itself, the earth fault path may not involve a protective conductor at all (e.g. cut lead causes exposes live part which is touched by a person standing on the ground).
   
    

• More importantly, PD IEC/TR 62350 Guidance for the correct use of residual current operated protected devices (RCDs) for household and similar use says that a test between L and PE may not be a suitable test ... especially in conditions that you might find when testing a caravan (loads operating elsewhere in the installation supplying the caravan, either at home, on site, or at the caravan maintenance workshop. I've certainly come across the issues stated in that standard when testing RCDs in Mobile & Transportable Units using the "L to PE" method

I understand that there are differing views on this method where RCDs used for ADS - is it OK not to use the PE? Well, 643.7.1 does not preclude the use of the up/down method to check the device is operating to spec, and other means, e.g. Z_e+R₁+R₂ or Z_s, to determine the disconnection times are met taking into account the characteristics of the device?

What If the RCD or RCBO is referenced to earth using a fly lead?


Andy B.

Can`t we cheat that one in a similar manner?

At best it will only check the RCD itself is working under those conditions (Fake TNC-S) and not the actual conditions on site but hey ho better than not testing at all methinks

Different question same subject.


Tonight I nipped into a house where there’s an RCD tripping issue. I put a Megger earth leakage clamp meter around the tails and when I opened the jaws one of the RCDs tripped. Obviously the meter tails are upfront of the RCD, but I tried it another three times and each time the RCD tripped when I opened the jaws. I have not figure out why at the moment, I am going back to do more testing and I am assuming there’s a neutral to earth fault on a RCD protected circuit.


Any thoughts on why opening the clamp meter jaws trips the downstream RCD?


Andy B.