TN system - Max Zs Vs tripping time in the Presence of RCD protection

Al Muatasem Al Saadi said:

Bottom line:
Yes—you can comply via the RCD route on TN. Verify Table 41.5 (RCD) + 0.4 s disconnection, and keep the RCBO’s overcurrent protection in place. electrical.theiet.org+

And, don't forget, if you use a voltage-dependent RCD/RCBO with E2 or E3 marking, check with the manufacturer regarding suitability for ADS ("indirect contact").

You can’t “ignore Zs,” but on a TN system you may rely on the RCD part of an RCBO for fault protection (ADS) instead of the MCB curve—if you verify to the RCD criteria rather than the MCB tables. In BS 7671 this is allowed (TN: Reg 411.4.5 / 411.4.204); use Table 41.5 (RCD method) instead of Tables 41.2–41.4 (MCB method). electrical.theiet.org+1

What that means for your example (20 A Type C RCBO, 30 mA):

• Your measured Zs = 1.40 Ω is higher than the MCB-only limit (e.g., 1.09 Ω), so it fails the MCB route.

• But for an RCD, the criterion is essentially Zs ≤ 50 V / IΔn → for 30 mA, ≈ 1667 Ω (see Table 41.5). Your 1.40 Ω passes easily, so ADS is met—provided the RCD disconnects within 0.4 s for final circuits ≤ 32 A (which your 20 ms test already shows). electrical.theiet.orgProfessional Electrician

Don’t forget:

• Overcurrent (overload/short-circuit) protection must still be satisfied by the RCBO’s overcurrent part—RCDs don’t protect L-N faults. electrical.theiet.org

Bottom line:
Yes—you can comply via the RCD route on TN. Verify Table 41.5 (RCD) + 0.4 s disconnection, and keep the RCBO’s overcurrent protection in place. electrical.theiet.org+

related to what used to be called the "alternative method" which permitted disconnection times for final circuits that would otherwise require disconnection in 0.4s to be extended to a time not exceeding 5s.

Arguably that option is still there - tucked away under 419.3 - all the "alternative method" did, in today's language, was ensure that the c.p.c. itself met the requirement for a supplementary bonding conductor.

   - Andy. 

Chris Pearson said:

so was it the norm then to install it with MCBs

It seems supplementary bonding in 15th Ed went well OTT. 

Chris Pearson said:

My concern is what to do with it when upgrading to RCD protection. Is it better to leave it in situ (which I have done) or remove it?

It depends on what else has changed ... for example plastic pipes etc.

In general, there is probably little harm inside the premises, it's influence outside especially PME (e.g. garden tap)

gkenyon said:

As Lyle points out, supplementary protective bonding is not really useful for B curve and C curve circuit-breakers, but is useful for fuses and D curve circuit-breakers.

My interest in electrickery post-dates 15th Edn, but at home there is supplementary bonding everywhere (including the kitchen sink), so was it the norm then to install it with MCBs?

My concern is what to do with it when upgrading to RCD protection. Is it better to leave it in situ (which I have done) or remove it? If removal is an option, is there a risk from leaving some of it behind - there is so much of it that it would be difficult to be confident that it had all been removed?

Thanks lyledunn .

The reason for my response was that I do hear it commonly put forward that BS 7671 doesn't permit touch voltages exceeding 50 V AC ... and I wanted to make sure no-one thought protective bonding (main or supplementary) actually achieved that in all cases.

I suppose it's correct to say that BS 7671, for dry conditions, doesn't permit touch voltages exceeding 50 V to exist indefinitely for the faults it considers in the general rules.

Newfutile said:

I know BS7671 allows this, perhaps bad design was a poor choice of words.i would still argue that meeting disconnection times with the mcb is a better design,and have rcd for additional protection.

It was certainly the way prior to 2008

Newfutile said:

I always assumed that BS7671 permitted rcd for ADS so that TT systems can be used.

RCDs were used in TT systems prior to 2008, but in 17th and 18th Editions (BS 7671:2008, and BS 7676:2018 ... both with relevant amendments), Regulation 411.4.5 makes it clear that Regulation 411.4.4 can be satisfied by an RCD as well as an overcurrent protective device.

Actually my post was two part, the first line related to the provision of supplementary bonding but the second part related to what used to be called the "alternative method" which permitted disconnection times for final circuits that would otherwise require disconnection in 0.4s to be extended to a time not exceeding 5s. The method did not require supplementary bonding but, of course, the cpc impedance was derived from  the final circuit protective device disconnection times plotted against the touch voltage curve. 

It was a feature of the 16th (413-02-12) and  the maximum resistance of the final circuit cpc was set out in Table 41C. Fuses were prevalent in final circuits back then.

I wasn't recommending the method to be deployed, just mentioning it out of interest and, to some extent, as an acknowledgement of how we have become slaves to look-up tables rather than understanding what we are trying to achieve. 

I know BS7671 allows this, perhaps bad design was a poor choice of words.i would still argue that meeting disconnection times with the mcb is a better design,and have rcd for additional protection.

I always assumed that BS7671 permitted rcd for ADS so that TT systems can be used.

Most remedial repairs I do are for failed rcd, probably because no one has ever pressed the test button.

lyledunn said:

method of limiting the volt drop along the cpc to 50v as referenced to the connection of the main protective bonding

This might sound very pedantic, and I mean no disrespect to lyledunn , but I feel it's important to correct a couple of points to avoid common misconceptions about supplementary protective bonding.

Using this method (Regulations 415.2 and 419.3), the voltage is not actually limited to 50 V for every possible fault.

It is limited to 50 V for currents up to and including the current which causes operation of the protective device (or, where multiple circuits share the same protective bonding, the largest of the protective devices) within 5 seconds.

Example: I have a B32 circuit-breaker to BS EN 60898. The current guaranteed to cause disconnection time within 5 s is 160 A, therefore supplementary protective bonding at the limit specified in Regulation 415.2 is guaranteed to limit to 50 V for fault currents up to 160 A. For fault currents exceeding 160 A, the touch current will exceed 50 V, but the circuit will be disconnected within 5 s.

Certainly, where supplementary protective equipotential bonding is applied in cases where a disconnection time < 5 s cannot be met (e.g. 0.4 s), but a higher disconnection time of 1 s or 5 s could be met, the touch-voltage is not guaranteed to be limited to 50 V.

As Lyle points out, supplementary protective bonding is not really useful for B curve and C curve circuit-breakers, but is useful for fuses and D curve circuit-breakers.

lyledunn said:

thereby allowing extension of disconnection times to 5s

This is also not the case. There is no maximum disconnection time to be applied for protection against electric shock  where supplementary protective bonding is applied according to all of the requirements of BS 7671 (at least in "dry condition"). This is because, for currents below that causing 5 s disconnection time (i.e. leading to longer disconnection times), the touch-voltage does not exceed 50 V by virtue of the formula in Regulation 415.2.

Disconnection within a certain time for protection against overcurrent (in this case fault current) may, however, still be necessary dependent on the characteristics of the overcurrent protective device for the circuit, and the cross-sectional areas and materials of the line conductor and cpc.