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EV charging on a domestic TT system.

IronFreely

This might have been chewed around a bit. 
So from a BS7671 point of view there’s a number of risk assessment issues surrounding EV chargers and the Earthing system for the installation and it’s proximity to other types of earthing system. So let’s just say you’re the luckiest sparkie ever, you’re installing an EVC on a farm miles from anything, the earth rod has a surprisingly low resistance to earth and off you go.

so my question arises from the need to protect all final circuits with a 30mA RCD on a TT installation. Now I’ve got no problem with the additional cost of installing a little dedicated unit to supply just the charger incorporating a 30mA RCD of the B variety, however the problem arises with selectivity as I am yet to find an EVC designed for the domestic sector that doesn’t include its own 30mA RCD as part of the charger. Contacting a popular manufacturer I asked if the RCD could be removed from the charger and installed upstream, the short answer “no”, the long answer “goodbye warranty”.  Obviously with this in mind I might be pushed towards a 100mA Stype up front but this still posses a problem in many installations especially with a TT system where a 30mA RCD is realistically going to be required upfront in the consumer. With a PME installation I’d be inclined to have a 60898 MCB upfront, perhaps using SWA to protect the cable where necessary but that’s not going to be ideal for TT and neither is having two 30mA RCDs  one upstream and one down stream. Even with TN installations there are going to be clients who insist on having the cable hidden within their paper thin walls, which again pushes me towards protecting that cable with a 30mA RCD.

So, what’s everyone’s thoughts, is it acceptable to ignore selectivity between RCDs where reasonably it’s possible to reset both if necessary, is 30mA RCD protection more important than selectivity ? Is there a technology solution, do the manufacturers need to catch up? Have I been corrupted by years of RCD dogma as a house basher and am missing an obvious trick? How do you approach a car charger on a TT system? 
I don’t mind TT installations myself, I’ve taken to installing RCBO boards and suitably protecting the incoming supply tails by way of double insulation and a compression tails entry gland designed for that exact purpose, my preferred brand of board also has optional tails clamps that go in above/behind the main switch for additional protection from movement. But still I’m faced with a selectivity problem.

Thanks in advance for your thoughts.

 

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    the need to protect all final circuits with a 30mA RCD on a TT installation.

    There's no blanket requirement for 30mA RCDs on final circuits - on TT systems or elsewhere (except possibly in some special locations). It's just the usual rules - typically just if you use soft sheathed cables concealed in walls etc. If it's just for ADS on a TT system then higher rated and time delayed RCDs are usually fine.

    I see no problem with two 30mA RCDs on the same circuit in principle - that's exactly what we have with caravans (one for the socket another inside the caravan). Good selectivity (or discrimination) is really about not loosing supply to other things unnecessarily - two RCDs on the same circuit don't disconnect anything extra which ever trips first (or even if they both do) - a single RCD supplying multiple circuits (as in the old split load CUs) is far worse in that respect, but is usually still acceptable.

    incorporating a 30mA RCD of the B variety,

    Be careful here - if a B-type RCD is required, then there must be a possibility of the residual current containing a d.c. component of over 6mA (which allegedly can affect the performance of A type RCDs) and a 30mA B-type RCD might not trip until the d.c.component reaches 60mA so doesn't protect upstream RCDs from that d.c. current either, hence all the upstream RCDs will have to be (minimum) B types as well. Often a better option is to use a charge point with an in-built RDC-DD (residual direct current disconnection device) that'll trip if the residual d.c. component reaches 6mA - thus not only can that circuit's RCD be an ordinary (and far cheaper) A type, but all the upstream ones can be too.

       - Andy.

Reply
  • 0

    the need to protect all final circuits with a 30mA RCD on a TT installation.

    There's no blanket requirement for 30mA RCDs on final circuits - on TT systems or elsewhere (except possibly in some special locations). It's just the usual rules - typically just if you use soft sheathed cables concealed in walls etc. If it's just for ADS on a TT system then higher rated and time delayed RCDs are usually fine.

    I see no problem with two 30mA RCDs on the same circuit in principle - that's exactly what we have with caravans (one for the socket another inside the caravan). Good selectivity (or discrimination) is really about not loosing supply to other things unnecessarily - two RCDs on the same circuit don't disconnect anything extra which ever trips first (or even if they both do) - a single RCD supplying multiple circuits (as in the old split load CUs) is far worse in that respect, but is usually still acceptable.

    incorporating a 30mA RCD of the B variety,

    Be careful here - if a B-type RCD is required, then there must be a possibility of the residual current containing a d.c. component of over 6mA (which allegedly can affect the performance of A type RCDs) and a 30mA B-type RCD might not trip until the d.c.component reaches 60mA so doesn't protect upstream RCDs from that d.c. current either, hence all the upstream RCDs will have to be (minimum) B types as well. Often a better option is to use a charge point with an in-built RDC-DD (residual direct current disconnection device) that'll trip if the residual d.c. component reaches 6mA - thus not only can that circuit's RCD be an ordinary (and far cheaper) A type, but all the upstream ones can be too.

       - Andy.

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  • 0 in reply to AJJewsbury

    First of lol thanks for your advice Andy. Yes indeed DC current and RCDs are not friends, I’m fairly confident that my Fluke tester uses DC current to keep the RCD closed when carrying out a live Zs test. My current thinking is that the manufacturers instructions  need to be referred to when selecting upstream RCDs, it’s probably safe to presume that when the charger itself incorporates an RCD with B type characteristics  that all upstream RCDs would also require B type characteristics, but RCDs not supplying the same circuits (for example a separate consumer unit serving another part of the installation) would be safe to remain AC, or these days A types (most over the counter RCBOs seem to have Atype characteristics, the ones I usually get do). 

    regarding ADS fault protection, correct me if I’m wrong but chapter 41 requires a 0.2 second disconnect for TT. I suppose I’m looking for a one size fits all solution and I think an MCB isn’t always going to give the required disconnection times, which just leads me straight back to an RCD. I seem to remember the book is a little vague and says that RCDs are preferred but an over current device is acceptable if disconnection times can be met, but fault current on a TT system can be very low, a bit better with bonding connected of course.

    I’m going to aim to discuss this on the course, I’m also going on an installers “course” with Zappi who are rocking the PME world with their technology, so I’ll discuss options with them too. 

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