Ring Continuity Test

[Jupiter01]

I know this stuff can get very technical, very quickly. Please go gentle as I am relatively new to this.

I am looking to spur from a socket on a Ring and want to check for continuity at this socket before I add the spur.

I am getting the following readings at this socket with my Megger:

R1 0.67
Rn 0.67
R2 0.78

It is using a 2.5mm T&E solid cable. I was expecting the R2 reading to be 1.67 times R1/Rn.

Given that this is lower, what could be causing this? Are there any simple checks I can perform? And does it really matter?

Thanks in advance.

 

[JohnW2]

I am getting the following readings at this socket with my Megger:
R1 0.67
Rn 0.67
R2 0.78

Where and how are you getting those measurements? What was 'disconnected'?

It is using a 2.5mm T&E solid cable. I was expecting the R2 reading to be 1.67 times R1/Rn.

One would think so, but if you went by the BS7671 ('voltage drop') figures, the ratio of resistance of 2.5mm² and 1.5mm² conductors is 29/18 = 1.61 -albeit not far from 1.67,and certainly much higher a ratio than you are seeing (about 1.16).

Given that this is lower, what could be causing this? Are there any simple checks I can perform? And does it really matter?

There could possibly be conductive paths in parallel with parts of the CPC - e.g. if there were metal conduit joining two or more backboxes which were 'earthed' to the circuit's CPC, or if the circuit was connected to anything whose 'earth' was in electrical continuity with metal pipework. Do I take it that there's no 'supplementary bonding' or any other connections to the CPC of the circuit that you are aware of?

Kind Regards, John

 

[Adam_151]

If for example your gas boiler or hob was on this circuit, then there would be an extra very low resistance connection from the spur/socket they are connecected at, through the pipework, the bonding cable back to the earth bar at the CU where that joins the CPCs for the cirucit you are working on, effectly bridging over that bit of the cpc of the ring.

If there was just the one, the effect would be eliminated if you tested at the CU instead of a socket as you haven't then got the second connection onto the CPC of your ring, but if you had multiple such things you'd still have the effect going on.

In practice its very hard to eliminate all the parralell paths, so generally don't wory about it. Theres various things that con result on parallel paths, such as metallic service pipes as above, metallic condtainment systems as noted by John and also metallic structural components, a lot of stud walls are built with metal studs these days, and often back boxes are attached with metallic braces https://www.cef.co.uk/catalogue/products/4362899-extendable-bracket-for-electrical-boxes-and-conduit . Its almost impossible to eliminate all parallel paths in such a situation

 

[Murdochcat]

Unless you have first and 2nd fixed said circuit the values are rarely by the text books.

I wouldn’t worry about the values and just note them on the MWC

 

[ebee]

Yes, if you`ve just wired/re-wired this ring final in the last few weeks then you might be able to evaluate where parallel earths might be affecting you readings, you might even decide to disconnect them if easy just in order to take a reading for assurance. With an existing older circuit you might decide just to make a quick evaluation mentally to see if you feel it is reasonable.

If I was to install a ring final I would do the r1, r2 & rn continuity tests before connecting any parallel paths if at all possible.

John, 2.5/1.5 is 1.66 recurring or 1.67 approx not 1.61 my friend

 

[EFLImpudence]

The VD figures are actually 29.333r and 17.6 which results in 1.666r - i.e. 5:3.

 

[EFLImpudence]

Anyway, why use the VD figures rather than simply 2.5:1.5 which will be (is) the ratio of the resistances?

 

[Jupiter01]

Where and how are you getting those measurements? What was 'disconnected'?

Getting these readings at one of the sockets I have unwired on this Ring. I did go round and unplug what I could see but may have missed something. I thought the unplugging was only mandatory with the IR test? Again, my ignorance could be to blame here.

which were 'earthed' to the circuit's CPC

Via a fly lead? If yes, they are used on my circuit. I need to do some more digging to see if there is any supplementary bonding at play.

John, I find your responses very helpful and well considered (often pitched at my level of understanding). Thanks again.

 

[Jupiter01]

If for example your gas boiler or hob was on this circuit,

Both are on this circuit and supplied through an FCU.

In practice its very hard to eliminate all the parralell paths, so generally don't wory about it.

Understood. How low does this R2 reading need to get before you should be concerned about it? Also, if it isn't parallel paths is there anything - more concerning - else that could be causing this issue?

 

[Murdochcat]

OP

do the tests at the CU with all the connections removed from the relevant places …..

 

[JohnW2]

The VD figures are actually 29.333r and 17.6 which results in 1.666r - i.e. 5:3.

Those are undoubtedly fairly 'correct figures, but where did you see them. Thwe figures in Appendix 4 are rounded to 29 and 18.

Anyway, why use the VD figures rather than simply 2.5:1.5 which will be (is) the ratio of the resistances?

I agree.

Kind Regards, John

 

[JohnW2]

John, 2.5/1.5 is 1.66 recurring or 1.67 approx not 1.61 my friend

I obviously know that, and acknowledged that such was what one would expect. I was merely pointing out (since I suspected that someone might otherwise try to get pedantic ) that the VD figures in App 4 of BS 7671 are rounded, such that the ratio appears to be about 1.61.

Kind Regards, John

 

[JohnW2]

Getting these readings at one of the sockets I have unwired on this Ring. I did go round and unplug what I could see but may have missed something. I thought the unplugging was only mandatory with the IR test? Again, my ignorance could be to blame here.

Things plugged in would only make a difference to the apparent resistance of the CPC if the CPC were connected,within the equipment, to something that could result in a parallel path, and even that only if there were more than one of them. It's hard to think of things that could do that - even WMs and DWs have their connections to metal pipework via plastic pipe and fittings.

John, I find your responses very helpful and well considered (often pitched at my level of understanding). Thanks again.

You're welcome - I do try

I've learned over the years/decades that when 'teaching' ('formally' or otherwise), one needs to carefully 'pitch' one's explanations so as to be appropriate for the audience (but one then has to be careful not to appear 'condescending' at times!).

Kind Regards, John

 

[JohnW2]

OP ... do the tests at the CU with all the connections removed from the relevant places …..

That won't prevent the apparent CPC resistance being affected by the sort of parallel paths that the rest of us have been discussing.

 

[JohnW2]

There's not really anything practical one can do about it, but one consequence of the issues being discussed above is that, in an existing installation, it is probably rarely, if ever, possible to be certain, by measurement, that the CPCs of the wiring are all intact and that the CPCs are all satisfactorily terminated.

There could be one or more breaks of the CPCs within cables (or their terminations) yet, if 'parallel paths' were present, one could get perfectly credible and reasonable 'continuity measurement' results.

Kind Regards, John