Max Zs entry on EIC for an RCD circuit

[equitum]

Interestingly, there is a section in the ECA's guide to the 17th entitled "the futility of testing Zs on an RCD protected circuit" or something very similar.

They say that RCD protected circuits do not have to meet the Zs requirements as the RCD guarantees that the required disconnection time is met. All that is needed is to test the RCD.

That's ridiculous. Here's why:

1. The regs demand circuits to be designed correctly: The breaker type would have to suit the EFLI, regardless of whether the circuit was RCD protected or not.

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.

Its not ridiculous, quite the opposite. Its correct. Heres why:

1. The regs state if the circuit is protected by an RCD the maximum Zs is determined by InR < 50V. There is no regulation that states "the breaker type must suit EFLI conditions" - nonsense. The breaker is selected in accordance with the designed load, breaking capacity vs PFC etc.

2. Nobody is saying dont test Zs to make sure that (for a 30mA device) that Zs < 1666 Ohm. If it "just happenned to be greater" then it would fail BS7671 and you wouldn't energise the circuit !

 

[securespark]

Nobody is saying dont test Zs

I think they are! See bold text.

Interestingly, there is a section in the ECA's guide to the 17th entitled "the futility of testing Zs on an RCD protected circuit" or something very similar.

They say that RCD protected circuits do not have to meet the Zs requirements as the RCD guarantees that the required disconnection time is met. All that is needed is to test the RCD.

There is no regulation that states "the breaker type must suit EFLI conditions" - nonsense.

So why do we have maximum EFLI charts which we use to ensure that the reading is acceptable for the CPD employed?

 

[Spark123]

You should never do an RCD test without first checking the efli anyway.

 

[equitum]

I'm sorry, didn't read that properly! Still stand by my points on Zs max being 1667 thoough (well, 1666.6666666666666666666666666666....) for 30ma rcd circuit

 

[securespark]

1. The regs state if the circuit is protected by an RCD the maximum Zs is determined by InR < 50V.

I have no access to my brown 16th at the moment. But my yellow one has:

RAIa [less than or equal to] 50V

under the heading "TT system".

It's not a general formula for any RCD-protected circuit.

 

[Spark123]

Reading the red book under TN systems, it allows the max efli value of 1667 for a 30mA device to BSEN 61008-1 and 61009-1 <32A, but then goes on to say in such cases an overcurrent device shall provide protection against overcurrent and fault current. Doesn't make sense to me!
For a protective device to function the efli of the circuit needs to be low enough to disconnect the protective device in the given time.

 

[equitum]

1. The regs state if the circuit is protected by an RCD the maximum Zs is determined by InR < 50V.

I have no access to my brown 16th at the moment. But my yellow one has:

RAIa [less than or equal to] 50V

under the heading "TT system".

It's not a general formula for any RCD-protected circuit.

Have a read of the current edition of BS7671 wiring regulations: Page 50.
411.4.9

 

[equitum]

Reading the red book under TN systems, it allows the max efli value of 1667 for a 30mA device to BSEN 61008-1 and 61009-1 <32A, but then goes on to say in such cases an overcurrent device shall provide protection against overcurrent and fault current. Doesn't make sense to me!
For a protective device to function the efli of the circuit needs to be low enough to disconnect the protective device in the given time.

Makes perfect sense to me, the RCD is providing FAULT protection. The OPD is providing protection against OVERCURRENT and FAULT current.

Fault protection and fault current are two entirely different things (see definitions, page 24)

 

[securespark]

Nothing you have written contradicts what Spark 123 wrote.

 

[Spark123]

Fault protection = new name for protection against indirect contact ( in this case Automatic Disconnection of Supply).
Fault current = current resulting from a fault.
Fault = A condition in which current current flows through an abnormal or unintended path......


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?

 

[equitum]

Nothing you have written contradicts what Spark 123 wrote.

Reading the red book under TN systems, it allows the max efli value of 1667 for a 30mA device to BSEN 61008-1 and 61009-1 <32A, but then goes on to say in such cases an overcurrent device shall provide protection against overcurrent and fault current. Doesn't make sense to me!
For a protective device to function the efli of the circuit needs to be low enough to disconnect the protective device in the given time.

Spark123 says the regs don't make sense. I say they do: That is a contradiction.

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.

 

[GaryMo]

Thats why EFLI is raised to 1667, as per table 41.5, and 411.5.3 (i) and (ii) being met.

Doesn't that relate to TT systems?
411.5 is headed as 'TT System'.

 

[Spark123]

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.

 

[equitum]

Thats why EFLI is raised to 1667, as per table 41.5, and 411.5.3 (i) and (ii) being met.

Doesn't that relate to TT systems?

411.5 is headed as 'TT System'.

411.5 is indeed headed TT systems.

However, 411.4.9 (at the top of page 50) is part of 411.4 - TN Sytems.

It refers to table 41.5, which incidentally is within "TT system". None the less it is table 41.5 which states the max Zs for a circuit protected by a 61008/61009 RCD <=32A is 1667 ohms .

 

[securespark]

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?