Max Zs entry on EIC for an RCD circuit

[equitum]

So the fault can be phase to earth and still needs to be dealt with by the fuse/MCB? If this is the case then the max efli will still be governed by the fuse/mcb surely?

No, the phase to earth fault is dealt with by the RCD. This will disconnect less than 0.4 seconds, even at 5xIn. Thats why EFLI is raised to 1667, as per table 41.5, and 411.5.3 (i) and (ii) being met.

But reg 411.4.9 calls for fault current to be dealt with by the overcurrent protective device, a fault causting fault current to flow can be phase to earth.
An RCD is not an overcurrent protective device.

I know that an RCD is not an OPD.

Spark123, yes, a fault causing fault current can be phase to earth. I accept that. I thought it pretty clear that the RCD is providing protection against a phase to earth fault.

Please bear with me and read 411.4.9, then chapter 43 (for the requirements of the OPD) to which it refers, and then the definitions again, study table 41.5 and its associated notes, and then maybe you will
agree with me. good fun this eh ?

 

[equitum]

You're confusing me...

You say 41.5 is under the heading of TT systems, but you say this 1666 figure relates to TN systems as well?

Yes. Its just that the table is in a "TT section" but it is refered to from a TN section. perhaps not the best idea, but that's the joy of BS7671 !!

 

[Spark123]

Please bear with me and read 411.4.9, then chapter 43 (for the requirements of the OPD) to which it refers, and then the definitions again, study table 41.5 and its associated notes, and then maybe you will
agree with me. good fun this eh ?

Sort of getting there, the notes to 41.5 only apply to TT systems . It is just the final sentance of 411.4.9 referring to fault current which I find confusing. OK, a fault may also be P-N or P-P I agree. It also seems to confuse a lot of other people that a table referring to TT systems is referred to in the TN section.

 

[securespark]

a table referring to TT systems is referred to in the TN section.

But is it? Or is that just eq's interpretation of it?

The thot plickens....

 

[Spark123]

Yeah, table 41.5 is in section 411.5 which is TT systems
411.4.9 is in section 411.4 which is TN systems.
Personally, if I ever saw an efli of anywhere near 1666 ohms on a TN system I'd be looking for the fault.

 

[securespark]

Found a Ze of 1500 and something on a TN-C-S supply once and nearly ****ed myself!

My personal highest was 1936 on a TT supply...with no RCD.

 

[RF Lighting]

My personal highest was 1936

I didn't realise you were quite that old

 

[securespark]

Ey up, Rob, leave off!

That's when my Mum was born.....

 

[equitum]

a table referring to TT systems is referred to in the TN section.

But is it? Or is that just eq's interpretation of it?

The thot plickens....

Surely not my interpretaion, just fact. (backed by the NICEIC)

The thing is, one section of the regs is referring to data in a table in another section of the regs. I assume its to prevent duplication. If you divorce yourself from the issue of where the table is, thens its not really that confusing, Everything else is logical, just use the values that are really only an application of V=IR < 50V
......... in my opinion. (sorry holmslaw for the "o" word)

typos and mispolings sponsered by carling

 

[davelx]

Good discussion chaps/chappesses!

I thought it would provoke some comments, but this is great.

Just for reference, it's para. C 4.6.5 on page 63 of the ECA book:

"C4.6.5 Irrelevant ELI specification

A common source of misunderstanding is that of either specifying or measuring values of ELI where the circuit also has an RCD fitted. ELI measurement under these circumstances is a futile exercise. The circuit will have been checked for continuity , and that is all that is needed together with, of course, functional checks of the RCD. This criterion satisfies requirements for automatic disconnection. The subject is somewhat confused by the inclusion of RCBOs in BS7671 Table 41.3 and for clarification, circuits fitted with RCBOs do not need to meet the specified ELI values."

 

[securespark]

So, back to my original comment:

2. A 30mA RCD is only good up to 1666 Ohms. What if you tested the RCD which was fine but you did not test the Zs, which just happened to be >1666 Ohms?

Then the RCD would not be guaranteed to give the required protection.

 

[equitum]

So, back to my original comment:

2. A 30mA RCD is only good up to 1666 Ohms. What if you tested the RCD which was fine but you did not test the Zs, which just happened to be >1666 Ohms?

Then the RCD would not be guaranteed to give the required protection.

In the ECA quote, it does mention testing for continuity. Typically when testing for continuity in say an R1+R2 fashion, 1667 Ohm may be deemed too high or raise alarm bells !

Thing is that the above ECA artical is not to be read as being the Regs. The regs, as I have demonstrated are quite clear that 1667 is the max value and should be entered as such on the EIC.

And this value should be measured against.

 

[securespark]

We'll have to agree to disagree on this one, matey!

 

[equitum]

We'll have to agree to disagree on this one, matey!

On what basis/evidence/fact can you possibly disagree having read the appropriate regulations ?

 

[securespark]

1666, not 1667, should not be the figure entered on EIC's for RCD-protected circuits.

Where does it say that in the regs?

(1667 exceeds 50V, me old china).