Ring Main - limits on length

[JohnW2]

Yes I used Ct = [230 + Tp - {(Ca² x Cg² - {Ib²/It²}) x (tp - 30)}] / (230 + Tp) in my Java Script program to get the correction factor.

Fair enough. [in the interests of avoiding ambiguity, I've added a few brackets (in red) to the above].

It's actually quite complex. What you have done (assuming a {method 100} It of 42A for the two legs of the ring together) is fair enough for loads at the midpoint of the ring, such that current will be shared equally between the two legs and will flow along the full length of both legs. However, since the 'guts' of the temperature correction (Ib²/It²) is not a linear function of current, the calculation will presumably not hold exactly for other arrangements of the loading.

We can argue about the technical merits but questions like is a RCD an over current device may be good fun in a collage class room but in real terms does the DIY man need to know or care. To me it is an over current device can't see how it can be classed as anything else. The fact that the current measured is the differential rather than current used does not change the fact it measures current.

Hmmm. That might be a good one for a debating society or in a pub with a glass in one's hand but, in the real world, it is only likely to cause total confusion! An RCD is not an "over-current device" in the sense that the term is nearly always used, or usually defined. There is clearly a need to distinguish between devices which operate in response to the current in L (and/or N) and those which operate in response to the current difference between L and N - and we do that at least partially by calling the former, but not the latter, "over-current devices".

Kind Regards, John

 

[ericmark]

The debate as to if a RCD measures and trips when the current exceeds a set limit i.e. over current is really down to "314.1 Every installation shall be divided into circuits". If you are correct then with a duel RCD board you may have 12 circuits, but if I am correct you only have 2 circuits and one can debate if it permitted to use twin RCD's or if RCBO's should be used.

Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by
the same protective device(s).

I did think you are correct in that a RCD is not considered as an over current device, until 2008 and it was made plain with (iv) that it was required have more than one RCD in a regulations talking about division into circuits, so because of what is inferred with 314.1 I now have changed my view and consider the RCD to split the supply into two or more circuits in most houses.

With the old 100 mA RCD it was rare for the RCD to trip, so there was no need to have more than one to minimize inconvenience in the event of a fault or any of the other sub sections. I will admit BS 7671:2008 is not an easy read. 431.1.2 for example says you don't need to switch the neutral when a RCD is used but in a very round about way. I some times think the document was designed to start arguments?

 

[JohnW2]

... "314.1 Every installation shall be divided into circuits". If you are correct then with a duel RCD board you may have 12 circuits, but if I am correct you only have 2 circuits and one can debate if it permitted to use twin RCD's or if RCBO's should be used.

Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).

I did think you are correct in that a RCD is not considered as an over current device, until 2008 ....

I really can't agree with you. Quite apart from the fact that you are taking "over-current device" to mean something different from what virtually everyone else means by the term, the BS7671 definition of "a circuit" does not use that exact term, anyway - as you have quoted, it talks about an assembly of electrical equipment "protected against overcurrent" by the same device - and I really don't think one can sensibly suggest that an RCD protects equipment "against overcurrent", can one?

If you were right and you were completing a Schedule of tests for an EICR or EIC (which requires one 'row of the schedule per circuit), would you (in the example you quote) really try to put the whole installation on two 'rows' of the Schedule (your '2 circuits'), rather than doing what everyone else does and put each of the 12 final circuits on a separate row.

Whilst one can argue about 'what the words are actually saying', I really don't think that your suggested interpretation is in any way useful/helpful, or that it makes any particular sense.

... and it was made plain with (iv) that it was required have more than one RCD in a regulations talking about division into circuits, so because of what is inferred with 314.1 I now have changed my view and consider the RCD to split the supply into two or more circuits in most houses.

I agree that 314.1.1 is not written very clearly, but I think you are probably taking it too literally. Per above, I think what 314.1 is talking about dividing an installation into groups of circuits (which makes sense) - and, in terms of normal English usage, "dividing into circuits" can mean "dividing into groups of circuits".

Kind Regards, John

 

[EFLImpudence]

Surely it would be more correct to say an RCD is an under-current device.

It detects when the neutral current is less than the line.

Before anyone says it could also be the other way round - no, the line is the current flowing, it is the neutral that changes because of a fault.

 

[JohnW2]

Surely it would be more correct to say an RCD is an under-current device. It detects when the neutral current is less than the line.

Hmmm - much as one can attempt to produce these semantic arguments, I think that would be just as confusing/unhelpful/'silly' than calling it an 'over-current device! We all (with the possible exception of eric - and possibly yourself, although I think your tongue is in your cheek!) know what is meant by an 'overcurrent device' and what is meant by a 'residual current' device, so why these attempts to confuse the situation, thereby potential frustrating communication?

Before anyone says it could also be the other way round - no, the line is the current flowing, it is the neutral that changes because of a fault.

That's true in the case of L-E and N-E faults on one single-phase circuit. However, in the case of cross-connections between circuits, it can be 'the other way around'. I also wonder what you'd have to say about a TPN RCD! Furthermore, an RCD (certainly a purely electromechanical one) does not know which conductor(s) are line/phase and which is neutral - all it knows about is about vector current differences between the conductors is is presented with.

Kind Regards, John

 

[stillp]

An overcurrent is a current exceeding the rated value, for a conductor the CCC of that conductor. So, unless your circuit has a rated value for differential current, an RCD cannot be considered an overcurrent device.

 

[bernardgreen]

It detects when the neutral current is less than the line.

In most cases that is true but there can be situations where there is current in the Neutral and nothing in the Live that will cause the RCD to trip.

It will occur when the network Neutral is a volt or two above ground and there is a Neutral Earth fault in an appliance that also has metallic connections to ground. It could also be a Neutral to back box fault where the back box in a damp wall close to ground level.

The majority of the current will divide and flow through the Main bond ( orange ) and the appliances CPC ( light green ) returning to the sub station via the ground ( dark green ). Some however will flow through the RCD on the Neutral, then through the fault to ground via the extraneous metal and back to the sub station. If the fault is a dead short then each path ( Neutral, CPC and Main bond wll each carry about 1/3 of the current.

 

[JohnW2]

An overcurrent is a current exceeding the rated value, for a conductor the CCC of that conductor. So, unless your circuit has a rated value for differential current, an RCD cannot be considered an overcurrent device.

Quite so. I know that I sometimes get involved in 'semantic debates' but, IMO, this one is just plain silly. As I've said we (virtually!) all know what is, and what is not, an 'overcurrent protective device" - and an RCD clearly is not one of them!

Kind Regards, John