RCD as 'sole' provider of ADS in TN systems

[JohnW2]

In this thread....

I think we discussed this one recently, didn't we lads? ... The answer was that BS 7671 permits the use of an RCD on circuits within TN installations where the Zs cannot be achieved, I believe. It must be the designer to decide what that means and what are the risks associated with relying on an RCD to trip within the required time period are, if any.

We have often discussed this, but I would hesitate to go as far as saying that we ever arrived at ‘the answer’.

In most senses, BS7671 appears to be saying that it is acceptable to rely upon an RCD for ADS in TN systems:

1...A footnote to Table 41.1 in 411.3.2.2 (disconnection times) says (without qualification about supply type) “Where compliance with this regulation is provided by an RCD ...”.

2...411.4.4 (specifically about TN systems) says that fault protection may be provided by an OPD or an RCD, adding (reasonably!) only that if an RCD is used for fault protection, the circuit must also have an OPD.

3...531.2.8 indicates (without qualification about supply type) that if an RCD is used for fault protection, it should be capable of coping with the possible fault currents.

4...As you have said, 531.3.1 indicates that an RCD may be used for fault protection in a TN system if 411.4.5 (essentially low enough Zs) cannot be achieved. However, it does qualify this as applying to “certain equipment in a certain part of the installation”. It is therefore questionable as to whether this could be invoked in relation to, say, a sockets circuit (since “certain equipment” would then not be defined).

415.1.1 and 411.3.3 speak (without qualification about supply type) of use of RCDs as ‘additional protection’. 411.3.3 speaks (without qualification about supply type) of an RCD as ‘additional protection’, and requires them on certain circuits (e.g. most domestic socket circuits), but does not specifically say as to whether the RCD can be ‘relied upon’ for ADS in TN systems (although some people may feel that ‘additional’ means that it cannot be the sole protection). .

The main reg cited by those who believe that, despite (1)-(4) above, an RCD cannot be be ‘relied upon’ for fault protection (i.e. ADS) is 415.1.2. This says that “the use of an RCD is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414”. However, particularly given that one of these measures (413) is ‘electrical separation’, I cannot help but wonder whether those people who cite this reg as prohibiting an RCD as the primary provider of ADS in a TN system are not perhaps ‘over-interpreting’ the reg.

Although the regs don’t always make this clear, it is obvious that with a TT system one nearly always has to rely on an RCD to provide ADS (i.e. ‘disconnection times’).

As for TN systems, I don’t know. I have to say that, taking the totality of the above into account, I personally find it fairly hard to believe that the regs are prohibiting reliance on an RCD for ADS. The question is obviously moot unless Zs is too high for the OPD to provide satisfactory disconnection times and, in that situation, 531.3.1 allows reliance on an RCD to provide fault protection for “certain equipment in a certain part of the installation”. Whether that includes things like domestic sockets circuits is seemingly open to debate.

Given the fallibility of devices, in some senses it would seem better to ‘rely upon’ an RCD for fault protection, since it can be tested, rather than to rely upon an OPD, which effectively can’t be tested.

What do others think?

Kind Regards, John

 

[Spark123]

Personally I would not like to rely on an RCD with electronics in it for fault protection in a TN system as in a short circuit the supply for the electronics may collapse hence the reasoning of ensuring the loop impedance is low enough for the over current device to do its job.

 

[JohnW2]

Personally I would not like to rely on an RCD with electronics in it for fault protection in a TN system as in a short circuit the supply for the electronics may collapse hence the reasoning of ensuring the loop impedance is low enough for the over current device to do its job.

Yes, we discussed that recently. For a start, I am being told (at least, by Bernard) that there are some RCDs which do not use electronics (just a direct trip of a 'precision mechanical mechanism' by current from the sense coil) - would you be happy to rely on one of those?

If the RCD does contain electronics, then I suspect that the fault would have to be very close to the RCD (obviously not impossible) to stop it working. The electronics probably only needs a few volts between L and N, and (at the very high fault currents involved) that would normally be provided by the voltage drop between the fault and the RCD. However, I agree that it's a theoretical issue.

However, I was not so much writing about whether reliance on an RCD is necessarily a good idea but, rather (given all the discussion), whether or not the regs allow it in a TN system. What is your view?

Kind Regards, John

 

[ericmark]

Personally I think we can rely on the RCD for disconnect times. However I can't quote a regulation to support this view.

 

[mfarrow]

Personally, I would prefer protection by cartridge fuses, rather than anything containing moving parts, which are always susceptible to 'stiction' and, in MCBs, poor selectivity at the cheap end of the market, as evident by 'guess which trupped first?' threads on here. Unfortunately we no longer live in such ideals thought about by the creators of BS 1361 and BS 1362, and mechanical trip mechanisms are unavoidable.

Electronics however should remain stable for a long period, and I would trust the electronics in an RCD no less than I would an MCCB with electronic trip unit, or an electronic protection relay. There is also a good reason why everything in life, from the clock on the wall to the fuel control in your car's engine, has become electronic in recent years.

 

[Taylortwocities]

Personally I think we can rely on the RCD for disconnect times. However I can't quote a regulation to support this view.

IIRC from my 236 learning. RCDs are one of the methods of achieving the required trip times (together with increasing the css etc etc).
But like you I don';t see a reg to match it.

 

[JohnW2]

Personally I think we can rely on the RCD for disconnect times. However I can't quote a regulation to support this view.

As I described at the start of this thread, there are several regs which appear to imply this, and really only one (about which I'm far from convinced) which some people seem to think indicates that this is not permitted.

Kind Regards, John

 

[JohnW2]

Personally, I would prefer protection by cartridge fuses, rather than anything containing moving parts, which are always susceptible to 'stiction' ...

I can't really disagree with that as a concept - but it clearly is not the direction in which we have been moving, and continue to move, and nor do I see any U-turns on (or beyond) the horizon!

Electronics however should remain stable for a long period, and I would trust the electronics in an RCD no less than I would an MCCB with electronic trip unit, or an electronic protection relay.

Indeed and, as I've debated with Bernard in other threads, I would personally be far more likely to trust the long term stability of an essentially electronic mechanism than long-term stability dependent upon a 'precision mechanical mechansim'. However, there is the issue already raised in this thread that a fault sufficiently close to the device theoretically might reduce the L-N pd to such a low level as to render the electronics inoperative. Although an adequately large capacitor across the power to supply to the electronics will keep the electronics powered for long enough to trigger the SCR, whether there will then be sufficient voltage (hence current) available for the trip solenoid to operate is, I would have thought, less certain.

Kind Regards, John

 

[mfarrow]

whether there will then be sufficient voltage (hence current) available for the trip solenoid to operate is, I would have thought, less certain.

I would hope so. But that's up to the standard owners to debate; we're just here to debate the standard.

 

[JohnW2]

whether there will then be sufficient voltage (hence current) available for the trip solenoid to operate is, I would have thought, less certain.

I would hope so. But that's up to the standard owners to debate; we're just here to debate the standard.

I don't think it's as simple as that. Writers of Standards can't change physics. If we were talking of a fault of negligible/zero impedance (beloved of writers of Standards and regulations) from the L load terminal of the RCD to a nearby MET of, say, a TN-C-S system, then I really can't believe that there would be enough voltage available between L and N of the RCD to allow the trip solenoid to operate - no matter what the Standard may like to think! Do you?

Kind Regards, John

 

[mfarrow]

I really can't believe that there would be enough voltage available between L and N of the RCD to allow the trip solenoid to operate - no matter what the Standard may like to think! Do you?

BS EN 61009-1 lists 4 classifications of RCBO type according to their operation, mainly depending on line voltage:

4.1.1 (no marking)
Function independent on line voltage
Use for indirect and additional protection
Provide service continuity

4.1.2.2a) (marked E1)
Function dependent on line voltage
Not opening automatically if line voltage lost
Able to trip under hazardous circumstances (e.g. earth fault) arising due to failure of line voltage
Use for indirect and additional protection
Provide service continuity

4.1.2.1b) (marked E2)
Function dependent on line voltage
Opening automatically if line voltage lost
Not reclosing automatically if line voltage restored
Use for additional protection
Does not provide service continuity

Edited:
4.1.2.2b) (marked E3)
Function dependent on line voltage
Not opening automatically if line voltage lost
Not able to trip under hazardous circumstances (e.g. earth fault) arising due to failure of line voltage
Use for additional protection
Provide service continuity

So there you have it, I suggest you e-mail your RCBO manufacturers to find out what you've got![/b]

 

[JohnW2]

I really can't believe that there would be enough voltage available between L and N of the RCD to allow the trip solenoid to operate - no matter what the Standard may like to think! Do you?

BS EN 61009-1 lists 4 classifications of RCBO type according to their operation, mainly depending on line voltage: <details> So there you have it, I suggest you e-mail your RCBO manufacturers to find out what you've got!

Interesting and intriguing. Many thanks. However, without more details, I have some difficulty in interpreting most of that; maybe you might be able to clarify some of it? ...

In terms of the ‘ordinary’ RCBOs we are used to, we know they are not 4.1.2.1b (E2), since they do not open in response to power loss (which we would call ‘active’ for an RCD).

The functional descriptions of 4.1.2.2a (E1) and 4.1.2.2b (E3) appear to be identical, the only difference being that the former is “for indirect and additional protection” but the latter only “for additional protection”. Is there anything in the Standard which indicates what is the functional difference which results in these different stated ‘uses’?

Does the standard clarify what is meant by “Able to trip under hazardous circumstances (e.g. earth fault) arising due to failure of line voltage”. Is this perhaps merely a reference to the fact that (given a low enough Zs) the overcurrent part (which probably does not involve any electronics) (but not the ‘RCD part’) will operate in response to a L-E fault even if there is no supply voltage, or does it mean something different or more complicated than that. In particular, does anything in the Standard indicate whether the ‘RCD part’ of the devices concerned (E1 and E3) still works when there is no power?

Do I take it that (although this would make one of them redundant) “Provide service continuity” actually means the same as “Not opening automatically if line voltage lost”? If not, does the Standard indicates what it does mean?

Given the fact that it has ‘no marking’ and a different section number, one suspects that 4.1.1 probably refers to ‘ordinary/common’ RCBOs but, ironically, few details about this one have been provided – is any more available? Is there any explanation of the meaning of “Function independent on (s.i.c.) line voltage”, since this is really the issue we’ve been discussing. In particular, does it really mean that the ‘RCD functionality’ will still work even if the L-N pd seen by the RCD falls to zero or near zero? If the answer is ‘yes’ then this leads me back to where I started, finding it hard to believe that a device using an electronics-driven solenoid could work under such circumstances.

Conversely (in case its one of those we’re interested in!) is there any further information about the meaning of “Function dependent on line voltage”. Does this simply mean that some of the functions (one might guess the ‘RCD’ ones) do not work in the absence of a supply, or does it indicate that function is dependent upon line voltage in some other, more complicated, fashion?

Any clarifications you can provide would be much appreciated.

Kind Regards, John

 

[JohnW2]

I really can't believe that there would be enough voltage available between L and N of the RCD to allow the trip solenoid to operate - no matter what the Standard may like to think! Do you?

BS EN 61009-1 lists 4 classifications of RCBO type according to their operation, mainly depending on line voltage: ....

Having just replied to this message at length, it's just occurred to me - why on earth are we talking about the Standards for RCBOs? This thread is about RCDs!!

Kind Regards, John

 

[JohnW2]

Personally I would not like to rely on an RCD with electronics in it for fault protection in a TN system as in a short circuit the supply for the electronics may collapse hence the reasoning of ensuring the loop impedance is low enough for the over current device to do its job.

Yes, we discussed that recently. .... If the RCD does contain electronics, then I suspect that the fault would have to be very close to the RCD (obviously not impossible) to stop it working. The electronics probably only needs a few volts between L and N, and (at the very high fault currents involved) that would normally be provided by the voltage drop between the fault and the RCD. However, I agree that it's a theoretical issue.

I've thought a bit more about this. The postulated problem (of L-N voltage at RCD falling to near zero, hence disabling electronic operation) could only arise with an L-CPC fault of negligible/zero impedance very close to the RCD in a TN-C-S installation. However, in such a situation, the EFLI would virtually inevitably be low enough for an OPD to operate. Even if (very unlikley) the fault arose between the RCD and downstream OPDs, the cutout fuse would operate.

With a TN-S installation (just as a TT one), such a fault would not result in line voltage dropping to anywhere near zero (likely to be around U0/2, which I'm sure would be more than enough for the electronics/solenoid).

With TT, quite apart from the fact that one has no choice but to rely on an RCD, the line voltage would not fall much with such a fault - so no problem.

So, with TN-S or TT, there's really no risk of the voltage across the RCD falling (as a result of an L-E fault) enough to compromise RCD operation. With TN-C-S there is theoretically a risk if the fault is very close to the RCD, but then an OPD will virtually inevitably operate. In practice, even with with TN-C-S, an L-E fault a significant distance from the RCD will still leave enough voltage across the RCD for its electronics to work - with a fault current of, say, 500A, and R1+R2 (from fault to RCD) of just 0.1&#937; would leave 50V across the RCD.

So, I don't really think there's really anything to worry about with electronically-operated RCDs. The electronics will nearly always get enough voltage to work and, in the one possibly exceptional case with an improbable fault (L-CPC fault very close to RCD) in a TN-C-S system, an OPD would virtually always operate.

Any comments?

Kind Regards, John

 

[ban-all-sheds]

Having just replied to this message at length, it's just occurred to me - why on earth are we talking about the Standards for RCBOs? This thread is about RCDs!!

But it's spun off from a thread about RCBOs.

Maybe mfarrow is to blame.