Hi, having a senior moment. What type of RCD fused spur wont trip the main consumers RCD if there is a fault on the RCD fused spur. We want to run a couple garden lights in & the ring main we are coming off has the fridges on that ring. As i remember there was a couple of makes that would let the RCD spur trip but not the main one. Any ideas what they are called & make if possible. Thanks.
The only way to do this would be for the house RCD to be a time delayed one. But you are not allowed to do that as it would remove the required protection for the 30mA-protected circuit(s). Your option in a regular house, would be to installa separate RCBO in a non-protected way in the CU and provide a separate circuit for the lights.
You can only achieve this if you use a local RCD with a tripping current way below that of the main RCD AND you take steps to limit the earth fault current to much less than 30mA. I am in aware of any product that will allow you to do this. If your garden lights are electrically separated, ie fed through a 12V transformer mounted inside the house, their is no requirement or need to add further RCD protection. The electrical separation offered by the transformer will prevent tripping of the main RCD in the event of an earth fault on the 12V lights.
Unless im going mad, i was sure there was an rcd spur that didn't trip the main box. I might be wrong but maybe a couple of diodes fitted may work, or something similar.
Greenbrook do a 10ma rcd fused spur. But as the other guys have said, it would be better on its own circuit with an RCBO.
As I have posted about this, quite a lot. So here we go again… A 10mA RCD downstream of a 30mA RCD will NOT provide discrimination. In the event of an earth fault of more than 30mA (a very common fault) it would be the RCD with the fastest trip time that will go first. That is not the way to protect your freezer full of frozen lobster! The ONLY way to provide RCD discrimination is with TIME, not current. That's what time delayed RCDs are for.
No RCD trips an RCD. It is a current imbalance in the circuit that trips an RCD. You really shouldn't be meddling if you are asking questions like this. Kind regards
Further to my previous post, I agree with the responders. The asker mentions the use of diodes in the RCD he remembers, this suggests that the earth fault current is returned as DC to the main RCD. But it is not the earth fault current that trips, it is imbalance between the 'live' conductors. I think their may be some confusion here between RCD and non trip testing for line earth loop using DC as the test current, as Robin did 10 years ago.
No problem, if this is what it takes, then it will have to be done the way suggested. But i do remember a sparks we worked with many years ago used a fused spur rcd that didn't trip the main board safety switch. Now just a a possibility, could this of been a ELCB back then.
Even time delay does not help, actual case, tea hut with 30 mA at 40 mS RCD, distribution board with 100 mA S type RCD, next board 500 mA with 30 seconds delay, and main board with 1 amp and 2 minute delay, site was building of Sizewell Power station so all RCD's had been tested on regular basis. Worker wanted to hang up his water proofs so put a nail in wall of tea hut which went through the cable, all 4 RCD's tripped, forget theory, this is what happened. With a leakage building up over time yes you may get discrimination, but not with a real fault like a 6" nail. So name of device which does not trip, it's called an isolator.
Somebody needs to do some testing on those RCDs then! The first RCD should have tripped within less than 300mSec. I would have thought that the nail would have provided a substantial earth fault, so the RCD would have tripped within 40mSec. A regular time-delay RCD shouldn't have tripped let alone the much slower (2 minute, really??) upstream time delayed RCDs.
Sorry mate. Just got in from work and completely misread it first time. See what you mean. Kind regards
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