Old wife's tail, myths, and other stories we were told as an apprentice, before the days when BS 7671 started.

[ericmark]

This is the thread where talk about TN-C-S problems are not going to cause problems, so yes go ahead, I have never worked out why PME was ever allowed?

Be it a hot tub, patio heater, or EV, the TN-C-S has been shown to be a problem. I can't understand why 50 volt unless an EV then 70 volt is permitted, yet back in the 70's told 25 volt can kill a cow.

In the 70's working as an auto electrician portable traffic lights were part of my job, new laws required traffic lights to be vehicle sensed, so I was sent off to learn how to repair these as then new traffic lights, the big change was the use of RADAR, well not quite RADAR but it was a detector which would see moving vehicles approaching it. The first versions "Peak" ran on 110 volt, but the replacement "Mullard" used 55 - 0 - 55 volt and would not work with our 110 volt generators.

It seemed they needed a centre tap, and in some way used the earth connection, which resulted in if the earth connection was lost, they could make the traffic light head 55 volt from true earth. Working for the council council we had their legal team to help, who told us 25 volt could kill a cow, so using the earth connection was not permitted, any supply used in the street where cows could touch the units, can't use the centre tap, so auto transformers to provide the centre tap had to be built into the traffic light heads. This was done at the expense of forest city who had upgraded the traffic lights.

As far as I know, cows have not been upgraded, so the rules of the 70's are still valid today, so any item on the street where cows can touch the device must not be able even in fault conditions to have more than 25 volt to true earth. Within fenced off compounds we may be allowed 50 or even 70 volt, but on the street 25 volt is the limit.

So is that an old wives tale, or true? And if true, what does it mean to EV charging points?

 

[Harry Bloomfield]

As far as I know, cows have not been upgraded, so the rules of the 70's are still valid today, so any item on the street where cows can touch the device must not be able even in fault conditions to have more than 25 volt to true earth. Within fenced off compounds we may be allowed 50 or even 70 volt, but on the street 25 volt is the limit.

So is that an old wives tale, or true? And if true, what does it mean to EV charging points?

The thing is, that animals are not more sensitive to electrocution, just that being larger than humans, their body and feet spacings, make them more susceptible to the voltage gradient.

 

[ebee]

yep a bigger wheelbase

 

[ebee]

Eric, that was very interesting, thanks

 

[ericmark]

Food for thought.

 

[securespark]

yep a bigger wheelbase

Made me laugh!

I use that expression regarding animals.

You're the only other person I know to do that!

 

[JohnW2]

I use that expression regarding animals. .... You're the only other person I know to do that!

Should it not be "hoofbase"?

 

[JohnW2]

yes John exactly so, to test Ze with bonds still attached could bring grief next week/month/year but some folk you cannot convince them.
and if no RCD as supplementary protection then blooming heck and hecky thump

True, but do not the 'guidelines' more-or-less 'require' one to measure Zs (or Ze) with main bonding still attached?

Kind Regards, John

 

[securespark]

Surely Ze is just the incoming earth on its own in isolation from all other conductors?

 

[JohnW2]

Surely Ze is just the incoming earth on its own in isolation from all other conductors?

Indeed it is, and so (the point I was making) what one measures with bonding connected may well be very different from Ze, but ...

... as I said, is it not the case (maybe my memory is wrong?) that guidelines 'require that Zs [which,after all, is simply Ze + (R1+R2) ] should be measured with main bonding connected ?

As you will realise, if you did that in my house, you would get figures for Zs [ i.e. hence Ze, if one subtracted (R1+R2) ] which were very very different from those one would get with the bonding disconnected.

 

[securespark]

You wrote "Zs (or Ze)", implying both are tested the same way.

One is in isolation, the other is not.

The theory does not always match test results.

I often had Zs figures lower than the theoretical figures.

 

[JohnW2]

You wrote "Zs (or Ze)", implying both are tested the same way.

As I said, they can be. One way of determining Ze is by measuring Zs and then subtracting (R1+R2) of the circuit.

One is in isolation, the other is not.

Maybe that's how people do it, but the only 'safe' approach would be to measure both 'in isolation' (i,e, with bonding disconnected) - since, if the Zs is different with and without bonding connected, (lower with bonding connected), then that is because of parallel paths through extraneous-c-ps, which cannot (should not!) be relied upon to always be present in the future (particularly as metal supply pipes are being progressively replaced by plastic).

Consider my house. If you measures Zs of any circuit with bonding connected, you would get results which indicated that the Zs was low enough for adequate fault protection to be provided by the circuit's OPD. However, if the waterpipe between my house and the neighbouring house were to be changed from metal to plastic, then the Zs of all circuits would rise to above 50, hence totally reliant upon RCDs for ADS. That doesn't matter so much these days, since nearly all circuits are required to be RCD protracted, anyway, but in the past it could have been potentially very 'dangerous' to measure Zs in my house with the main bonding connected (even though some people would do, and would have done, that!).

The theory does not always match test results. I often had Zs figures lower than the theoretical figures.

I'm not sure what 'theory' and 'theoretical figures' you are think of.

The only 'theory' I'm referring to is simple addition (or subtraction). If you obtained Zs figures which were lower than Ze plus (R1+R2), then something must have been wrong with one or more of your measurements/calculations.

Kind Regards, John

 

[ebee]

Yes folks,
The proper way to do it is
A/ Take the Ze reading without any bonds, that is the figure that is added to R1 + R2 in our calcs to ensure the circuit will comply.
B/ To take another Ze reading with all bonds etc in place, this is to get the figures of PEFC PSSC , it is also important.
Never one or the other but both, it is extremely important that you can demonstrate that your new circuit will work safely (A/) and your MCBs/Fuses/Switchgear can sustain the maximum let thru energy with failing for the required time.

When measuring A/ additional precautions are needed, you don't want yourself or anyone else at risk from unbonded installations during fault measuring procedures.

Of course if you are just measuring one item on an existing circuit (example an additional socket) you only do B/ usually to get the Z (Zs) of the addition, you are unlikely to need to do the test without bonds, this way you do the least risky option but hope that bonding etc is not falsifying your expectations. That in itself is one dammed good reason for doing EICRs (PIRs) at regular intervals otherwise you risk missing the fortuitus earthing effect of bonding which could be removed at any time without your knowledge.

Whilst as a purely mathematical equation Ze + R1 + R2 = Zs therefore Zs - Ze = R1 + R2 holds good.
We never ever ever calculate Zs - Ze = R1 + R2 in Electrics, it is a false equation and so must never be used.
I have mentioned before one switchfuse I encountered that had a Ze recorded on the PIR yet there was no earthwire from the supply. I reckoned someone had purely put the testgear on the switchfuse earthbar and not noticed it, all they were getting was a reading from the cpc of the shower cable at origin - numpties!

PS - for the pedantic - No, Zs does not equal Ze plus R1 plus R2 but using it is usually good enough for most domestic and a lot of other installations because it only measures Resistance but not Impedance

 

[JohnW2]

The proper way to do it is
A/ Take the Ze reading without any bonds, that is the figure that is added to R1 + R2 in our calcs to ensure the circuit will comply.
B/ To take another Ze reading with all bonds etc in place, this is to get the figures of PEFC PSSC , it is also important.
Whilst as a purely mathematical equation Ze + R1 + R2 = Zs therefore Zs - Ze = R1 + R2 holds good.
We never ever ever calculate Zs - Ze = R1 + R2 in Electrics, it is a false equation and so must never be used.

Your statements are self-fulfilling because IF (as you suggest) one measures Zs and Ze under different conditions (one with, and one without bonding in place), then the equation will not necessarily (in fact, rarely will) remain valid.

It remains my view that the only 'safe' way to measure Zs is with bonding disconnected, since otherwise one might get a result which indicates that Zs is low enough for OPD-mediated ADS, but with the 'potentially dangerous' situation (C2 on an EICR? ) that such might cease to be the case if paths to earth through extraneous-c-ps disappeared (or deteriorated) in the future

We never ever ever calculate Zs - Ze = R1 + R2 in Electrics, it is a false equation and so must never be used.

Hmmmm. It seems that my memory was not wrong Admittedly my copy of GN3 is quite old (and maybe has subsequentlyt changed??) but, in keeping with my recollection, it says ...

It is perhaps interesting that the first method of determining Zs they mention is not 'by measurement' but, rather, is by calculation as Ze +_ (R1+R2).

Furthermore, although they don't explain what they mean by determination of Ze "by calculation", the only thing I can think of is that they are talking about the 'determination of Ze' by subtraction of (R1+R2) from measured Zs ? If one did that, with the Zs having been measured with main bonds [present (as you suggest), then the calculated Ze would not correspond with the figure you define (without bonds), since it would/could include parallel paths via extraneous-c-ps.

GN3 does say the same as you, that Ze should be measured with no bonding (or any part of the installations earthing system) connected but that Zs should be measured will all bonding present, but then goes on to make the very point I've made above about the problem of this approach, when it says ...

I do somewhat struggle to see what they mean by "taking into account" the error which is likely to result from this approach - since, as I've said, one is at the mercy of unknown changes in parallel paths in the future. As I've said, the only really safe approach is to measure Zs with bonding disconnected, hence 'taking into account' the possibility that the parallel paths may totally and suddenly disappear at some point in the future.

I must say that, given these 'downsides' of measuring Zs with bonding connected, I'm not sure what perceived 'upsides' over-ride them (other, I suppose, than the safety of the person undertaking the test). Although you wrote '''

..... B/ To take another Ze reading with all bonds etc in place, this is to get the figures of PEFC PSSC , it is also important.
Never one or the other but both, it is extremely important that you can demonstrate that your new circuit will work safely (A/) and your MCBs/Fuses/Switchgear can sustain the maximum let thru energy with failing for the required time.

... The relevant PEFC for an installation presumably relates to a fault very close to the origin of a circuit, hence essentially determined by Ze. If (as we both agree) Ze is measured without bonding in place, then that figure will underestimate the PEFC when bonding is connected. In a TN-C-S installation, the same is true of PSSC. If one wanted a 'true' PEFC (or 'true' PSCC in a TN-C-S installation), I suppose one would have to also measure Ze with bonding present.

In summary, I don't think the situation is quite as simple/straightforward your assertions suggest

Kind Regards, John

 

[EFLImpudence]

It remains my view that the only 'safe' way to measure Zs is with bonding disconnected, since otherwise one might get a result which indicates that Zs is low enough for OPD-mediated ADS, but with the 'potentially dangerous' situation (C2 on an EICR? ) that such might cease to be the case if paths to earth through extraneous-c-ps disappeared (or deteriorated) in the future

Yes, that is why Ebee said Zs is calculated by adding Ze to R1+R2 - so that any bonding is discounted.