Supplimentry Bonding 17th Edition

[securespark]

As stated above, it is just about impossible for a circuit to exceed 1666Ω so we may as well not measure anything.

I agree with your sentiments about testing, but disgree about exceeding 1666...although it is not a weekly occurrence...

//www.diynot.com/forums/viewtopic.php?p=921726#921726

 

[ban-all-sheds]

A common source of misunderstanding is that of either specifying or measuring value of E[F]LI where the circuit also has an RCD fitted. E[F]LI measurement under such circumstances is a futile exercise. The circuit will have been checked for continuity, and this is all that is needed together with, of course, functional checks of the RCD. This criterion satisfies requirements for automatic disconnection."

They go on to state that this also applies to circuits protected by an RCBO.

According to some chap called Paul Cook:

In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.

 

[JohnW2]

A common source of misunderstanding is that of either specifying or measuring value of E[F]LI where the circuit also has an RCD fitted. E[F]LI measurement under such circumstances is a futile exercise. The circuit will have been checked for continuity, and this is all that is needed together with, of course, functional checks of the RCD. This criterion satisfies requirements for automatic disconnection."

I totally disagree with that. Perhaps the common source of misunderstanding is not where the author thought it was. Can you check for continuity without realising what the value of R1+R2 is?

Given that the electrical trade uses 'continuiity' to refer to a measurement of resistance, then that last statement is obviously correct, but I don't think the quoted statement is quite as silly as you seem to think, particularly in a TT system.

If one has measured Ze (once), and has found it to be acceptable (for the earthing system) and then measures R1+R2 for each circuit one can, indeed, go around adding Ze to R1+R2 for each circuit and get figures for Zs, or one could measure Zs for each circuit, and then record all these figures. However, assuming that all of these figures are below 1666&#937; (which we've all agreed is essentially inevitable, unless something very wrong is going on), then what purpose does this series of recorded figures serve? In many/most TT installations, those figures will all be far too high to provide required disconnection times with an OPD, so what do they indicate/prove? At the very least,one could argue that "<1666&#937;" would be adequate to record for each circuit.

If there were "something very wrong going on" (e.g.disconnected earth rod and minimal parallel paths to earth), only an idiot would go around measuring and recording a whole pile of Zs values way above 1666&#937;!!

Kind Regards, John.

 

[JohnW2]

According to some chap called Paul Cook:
In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.

I thought this was the alleged reason for the 'functional earth' connection of an RCBO - to enable the 'RCD mode electronics' to function even without a neutral. Mind you, straight RCDs do not seem to have addressed this issue, so the potential problem does still apply to them. Of course, with TT one has no choice but to rely on RCD functionality to meet disconnection times.

Kind Regards, John.

 

[JohnW2]

As stated above, it is just about impossible for a circuit to exceed 1666&#937; so we may as well not measure anything.

I agree with your sentiments about testing, but disgree about exceeding 1666...although it is not a weekly occurrence... //www.diynot.com/forums/viewtopic.php?p=921726#921726[/QUOTE]
Yes, but that 1936&#937; Zs you reported on a TT system was presumably due to a very iffy (or disconnected) earth rod. The point surely is that, once one has measured (once) the Ze on a TT system (and found it satisfactory), then the actual value of Zs for individual circuits is effectively irrelevant. If R1+R2 for any circuit were 1500&#937;+, that would be a totally different problem (which needed rectification) from "a high Zs"!

Kind Regards, John.

 

[securespark]

Sorry, John. I don't follow.

You say if the Ze is OK, the Zs is irrelevant. But then you say if the R values exceed 1500 Ohm, that would need rectifying?

 

[Spark123]

I thought this was the alleged reason for the 'functional earth' connection of an RCBO - to enable the 'RCD mode electronics' to function even without a neutral. Mind you, straight RCDs do not seem to have addressed this issue, so the potential problem does still apply to them. Of course, with TT one has no choice but to rely on RCD functionality to meet disconnection times.

Kind Regards, John.

Bog standard RCDs (or RCCBs) are made up of a series of windings around a torroidal core and a trip coil hence do not need an external supply to drive them, the operational current is derived from the current in the sensing circuit.
On the other hand, RCBOs tend to have electronic amplifiers in them that need a supply voltage.

 

[JohnW2]

Sorry, John. I don't follow. You say if the Ze is OK, the Zs is irrelevant. But then you say if the R values exceed 1500 Ohm, that would need rectifying?

I don't really understand your problem...

If the Ze is OK, then the Zs is bound to be adequate to provide satisfactory RCD operation with any credible/acceptable R1+R2 of any final circuit.

If the R1+R2 of a final circuit were greater than 1500&#937; (I chose that figure to allow about 167&#937; for a very poor TT electrode), then there is clearly something seriously wrong, and potentially dangerous, (and ridiculous in term of VD) with the cable run which needs immediate investigation and rectification. Although I said 1500&#937;, I could just as easily have said something like 10&#937; (about 200 metres of 1mm T/E).

Is that not fairly obvious?

Kind Regards, John.

 

[JohnW2]

Bog standard RCDs (or RCCBs) are made up of a series of windings around a torroidal core and a trip coil hence do not need an external supply to drive them, the operational current is derived from the current in the sensing circuit. On the other hand, RCBOs tend to have electronic amplifiers in them that need a supply voltage.

Ah! I must confess that I had thought that RCDs also had amplifiers. If what you say is correct, that explains the difference.

Why the difference, then? Is it perhaps so that RCBOs can use much smaller sensing toriods to enable them to fit into single module width enclosures?

Kind Regards, John.

 

[securespark]

I don't really understand your problem...
.................Is that not fairly obvious?

Kind Regards, John.

There's no need for that kind of response, is there?

Kind regards, Simon.

 

[securespark]

Read again what you wrote in response to my post:

Yes, but that 1936&#937; Zs you reported on a TT system was presumably due to a very iffy (or disconnected) earth rod. The point surely is that, once one has measured (once) the Ze on a TT system (and found it satisfactory), then the actual value of Zs for individual circuits is effectively irrelevant. If R1+R2 for any circuit were 1500&#937;+, that would be a totally different problem (which needed rectification) from "a high Zs"!

Kind Regards, John.

Then what you wrote in clarification:

If the Ze is OK, then the Zs is bound to be adequate to provide satisfactory RCD operation with any credible/acceptable R1+R2 of any final circuit.

If the R1+R2 of a final circuit were greater than 1500&#937; (I chose that figure to allow about 167&#937; for a very poor TT electrode), then there is clearly something seriously wrong, and potentially dangerous, (and ridiculous in term of VD) with the cable run which needs immediate investigation and rectification. Although I said 1500&#937;, I could just as easily have said something like 10&#937; (about 200 metres of 1mm T/E).

"The actual value of Zs is irrelevant"

This is the bit I was struggling with mainly.

Ze + ? =Zs

 

[JohnW2]

I don't really understand your problem... Is that not fairly obvious?

There's no need for that kind of response, is there?

I'm sorry if it offended you, but I didn't understand your problem and did think that it was fairly obvious. Surely no-one would deny that an R1+R2 of tens of ohms, let alone >1000&#937; indicated something seriously wrong with the circuit which required immediate rectification?

Anyway, as I said, if I offended you, then I apologise.

Kind Regards, John.

 

[JohnW2]

Read again what you wrote in response to my post:

Yes, but that 1936&#937; Zs you reported on a TT system was presumably due to a very iffy (or disconnected) earth rod. The point surely is that, once one has measured (once) the Ze on a TT system (and found it satisfactory), then the actual value of Zs for individual circuits is effectively irrelevant. If R1+R2 for any circuit were 1500&#937;+, that would be a totally different problem (which needed rectification) from "a high Zs"!

Sorry, but I really still don't understand your point/problem - maybe I didn't write clearly enough. Would it have been clearer if I had written something like:

Yes, but that 1936&#937; Zs you reported on a TT system was presumably due to a very iffy (or disconnected) earth rod. The point surely is that, once one has measured (once) the Ze on a TT system (and found it satisfactory), then the actual value of Zs for individual circuits is effectively irrelevant, provided that they do not indicate a serious problem with the circuit wiring. If R1+R2 for any circuit were 1500&#937;+, that would be a totally different problem (which needed rectification) from "a high Zs"!

What I was getting at is, once one has an installation with which is generally satisfactory (which would imply that any inappropriately high R1+R2 figures had been investigated and rectified), no useful purpose is seemingly served by recording a series of Zs figures of tens of ohms (or maybe ~100&#937 in a TT installation.

Kind Regards, John.

 

[securespark]

Therein lay my confusion.

Thank you.

 

[JohnW2]

Therein lay my confusion. Thank you.

I'm glad that all is now clear and, as I said, apologise if my initial response offended you. Perhaps a clearer way to express what I was trying to say would have been something like:

• "If Ze and R1+R2 are both reasonable/acceptable, then, in a TT installation, the actual value of the sum of them (i.e. Zs) (which is usually going to be too high to achieve required disconnection times with an OPD) is essentially irrelevant."

Kind Regards, John.