UK Selectivity : What does it mean specifically, especially in relation to EV chargers

[clanky]

Hi all, been reading around this and i have to admit ive quickly gone out of my depth as regards EV chargers. The charger in question is a Myenergi zappi which has its own internal RCD and PEN protection. So, the usual method it seems, is to fit another small consumer board and install a DP 32/40A (either a RCD/MCB/RCBO) with an SPD. Because of the zappi's internal rcd, it seems that an MCB is the preferred choice of protection for the cable to the charger due to selectivity. What does this mean exactly? and if an MCB is fitted, doesnt that mean the cable is only protected for overload and not for any earth faults? As always, thanks for the input.

 

[timhoward]

Your question could have a long essay written to fully cover the points you raise.
The quick version:
Selectivity - make sure only one RCD trips
MCB - if enough current flows it will trip. If the earth impedance path is low enough this includes current flowing to earth. (Usually TN earthing achieves this).
An RCD might be required to achieve this if there is no supplier earth (TT earthing)

The regs may also require additional RCD protection depending on the cable type and how it’s installed. E.g Twin and earth buried in a wall would need it.
That scratches the surface off - hope it helps.

 

[loz2754]

The internal RCD of the Zappi is not compliant with BS EN 61008, or 61009, so a Type A RCD/RCBO must be installed upstream.

 

[Electro-tech]

I think this is current .

 

[Risteard]

The internal RCD of the Zappi is not compliant with BS EN 61008, or 61009, so a Type A RCD/RCBO must be installed upstream.

And it must interrupt all live conductors, including the neutral conductor.

 

[pc1966]

The situation here is a bit different to the usual selectivity case. The classic one is you have some form of protection (OCPD and/or RCD) feeding some DB from which several other circuits all have their own protection and so the goal is to make sure:

• The feed to the DB is protected against faults and (probably) against overload
• As far as sensible, that a fault on any of those sub-circuits does not trip the up-stream protection and so impact the other healthy sub-circuits.

In the case of an EV charger it is almost certainly on a dedicated radial circuit and so under fault conditions having both trip is not a big deal - there is a fault to find & rectify anyway. So basically you treat the EV as any other outdoor device - the feed cable has to be protected against faults and RCD protection may be needed for that (TT case, etc) and may be needed due to the risk of a penetration accident (oh-er!) if it is not SWA or similar.

Off hand I don't know what the Zappi charger specifies, but I would fit anything like that with DP RCD protection (or neutral-switching RCBO) as you would any other outdoor unit.

The internal RCD is non-standard I believe because I think it is also part of the open-PEN protection so I guess it also switches the CPC (L+N+E) on detecting a fault, whether that is a classic imbalance on L-N or something indicating a flow of current in the CPC (e.g. open PEN has said car to high voltage and some poor sod is washing it with feet on true Earth).

EDIT: Just to add check the manufacturer's instructions as some EV chargers might need a type B "DC sensing" RCD to be used at the supply point.

 

[pc1966]

From the Zappi FAQ section:

• Circuit breaker or 30mA Type A RCBO depending on local installation requirements, 32A curve B. Please note that a higher rated MCB/RCBO may be needed due to thermal derating factors as recommended by the MCB/RCBO manufacturer.

 

[pc1966]

From the Zappi manual:

Supply Connection
The zappi device should be connected to a single-phase 230V or 240V nominal AC supply. The supply should be from a dedicated 32A or 40A circuit breaker.

zappi features and integral 30mA Type-A RCD, therefore an RCD protected supply is not required unless local regulations state otherwise.

 

[timhoward]

From the Zappi manual:

Supply Connection
The zappi device should be connected to a single-phase 230V or 240V nominal AC supply. The supply should be from a dedicated 32A or 40A circuit breaker.

zappi features and integral 30mA Type-A RCD, therefore an RCD protected supply is not required unless local regulations state otherwise.

Agreed, but this is one of those confusing and slightly annoying situations.
It's a case of so close yet so far.
The integral device is an RDC-DD to IEC 62955, NOT an RCD to IEC 61008 or an RCBO to IEC 61009.

So unfortunately, it doesn't comply with the letter of BS7671.
I think there's also an issue in that as it's an electronic device, a power cycle (or power cut and power restoration) will reset it without user intervention.

They have slightly amended the online info now, alluding to this:

EDIT to be fair @loz2754 already mentioned most of this in post #3

 

[pc1966]

As so the guidance notes they provide is actually wrong? What a surprise

 

[clanky]

Thanks guys for all your inputs. Was unfortunately away with work so couldnt interact. I shall work my way through the comments and ponder over the info. Be prepared for more queries!

 

[PeakSteve]

Hi all, been reading around this and i have to admit ive quickly gone out of my depth as regards EV chargers. The charger in question is a Myenergi zappi which has its own internal RCD and PEN protection. So, the usual method it seems, is to fit another small consumer board and install a DP 32/40A (either a RCD/MCB/RCBO) with an SPD. Because of the zappi's internal rcd, it seems that an MCB is the preferred choice of protection for the cable to the charger due to selectivity. What does this mean exactly? and if an MCB is fitted, doesnt that mean the cable is only protected for overload and not for any earth faults? As always, thanks for the input.

In a TN system, hopefully, Zs will be within limits. In a TT system, the Zs will be higher than permitted, so you have 2 choices: Either fit a (Type A) 100mA RCD upstream (the Zappi claims it has "Type A + 30mA DC" RCD protection, so Type A (just) complies with the same-type RCD rules), and use just a (32A) MCB, or - if the cable is "protected" (e.g. an SWA cable) you make it a (32A) "Distribution Circuit" (so MCB, but no RCD-protection).

I have discussed this with a number of NICEIC inspectors and thoughts vary on whether this approach actually constitutes a D.C. However, when I point out that the isolator switch, AND the Zappi are Class II, so there is NO Earth access this side of the Zappi, and that I could otherwise install an (un-necessary) 1-way distribution board with a 32A MCB in it, to make it an actual distribution circuit, they tend to agree.

 

[loz2754]

the Zappi claims it has "Type A + 30mA DC" RCD protection, so Type A (just) complies with the same-type RCD rules)

Actually this is not the case. See this excerpt from the MyEnergi website:

"It is important to note that the standards for EV charge point installation usually require an RCD which meets a specific standard – IEC 61008 for an RCD or IEC 61009 for an RCBO. The RCD in zappi does not fully meet these requirements which means that a separate RCD or RCBO to these specific standards needs to be installed."

 

[loz2754]

you make it a (32A) "Distribution Circuit"

It's still a final circuit, not a distribution circuit, just one that doesn't need additional protection by a 30mA RCD. Unless it feeds another consumer unit or DB of course. (As you stated).

And just to add that the regulations require EV charge points to be protected by a type A 30mA RCD that isolates all live conductors, including the neutral.

The regulations also refer to it being on a dedicated RCD, which some have interpreted to mean that the RCD cannot be shared with other circuits, as it would be in a split load consumer unit.

I personally find that in many cases, it's easier to supply an EV charge point from it's own small consumer unit, particularly if the main consumer has no SPD fitted.

 

[timhoward]

@PeakSteve if you read the whole thread there is more info about the Zappi and it's internal RDC-DD (not RCD). It's a right royal pain but it is what it is.