Touch voltage

[JohnW2]

Yes, I knew you'd get it in the end.

I didn't initially realise that you were talking about 'two faults' (L in contact with exposed-c-p and no CPC connected to it)!

Kind Regards, John

 

[JohnW2]

I'm going in the bath now. No exposed nor extraneous-c-ps in the room.

That's an interesting point. Not surprisingly, all of my bathrooms have things which are potentially extraneous-c-ps (pipes etc.) but none of my bathrooms, nor any of those I commonly frequent, have any exposed-c-ps (except, possibly, some above-zones, and certainly not touchable, light fittings) - which leads me to wonder (in view of all the discussions about SB etc.) how common it actually is for bathrooms to have any touchable exposed-c-ps?

Kind Regards, John

 

[EFLImpudence]

I didn't initially realise that you were talking about 'two faults' (L in contact with exposed-c-p and no CPC connected to it)!

To be honest I wasn't.

More trying to explain or work out (as in Studentspark's example) the unlikely situation where an earth fault, for some reason, takes a long time to clear or does not clear at all.

 

[EFLImpudence]

That's an interesting point. Not surprisingly, all of my bathrooms have things which are potentially extraneous-c-ps (pipes etc.) but none of my bathrooms, nor any of those I commonly frequent, have any exposed-c-ps (except, possibly, some above-zones, and certainly not touchable, light fittings) - which leads me to wonder (in view of all the discussions about SB etc.) how common it actually is for bathrooms to have any touchable exposed-c-ps?

I think most people who discuss the subject, usually forget about the 'simultaneously accessible'.

 

[JohnW2]

To be honest I wasn't.

Fair enough, but ...

More trying to explain or work out (as in Studentspark's example) the unlikely situation where an earth fault, for some reason, takes a long time to clear or does not clear at all.

... is not the answer that such will not happen if (assuming TN) the circuit has been designed to have an adequately low Zs and the exposed-c-p is still connected to the circuit's CPC?

Kind Regards, John

 

[JohnW2]

I think most people who discuss the subject, usually forget about the 'simultaneously accessible'.

That's very probably true but 'worse' than that, as I said, none of the bathrooms with which I am familiar have any exposed-c-ps at all - and I imagine that the same is true of many/most bathrooms (other than, in some cases, Class I light fittings on the ceiling that one would need a stepladder to be able to touch) - electrically-heated towel rails are perhaps about the most likely possessors of exposed-c-ps in a bathroom, but I can't think of any bathroom I know which has one of them.

Kind Regards, John

MOD: FYI The OP has left the conversation.

 

[EFLImpudence]

He'll be back.

We are discussing the same subject.

 

[JohnW2]

MOD: FYI The OP has left the conversation.

Eh? That seems to be very 'trigger happy'. He was still 'here' about three hours ago and, judging by what he then wrote, has gone away to think about what I wrote in my (then) previous post (and I strongly suspect 'will be back') ...

Thank you. I'll need to read it a few times. Really appreciate your help

Kind Regards, John

 

[studentspark]

MOD: FYI The OP has left the conversation.

On the contrary, the conversation has left me.......scratching my head

50/230 = R2 / (R1+R2)

after which it's just a little simple algebra ....

Indeed. I'm still working on it.

Thanks

 

[ericmark]

I also wonder about the risk, so much has changed over the years, but people still look back to distance past, I am sure today the risk to a monkey being found in Hartlepool and hanged as a French spy is very remote, but we still talk about it.

There is no point where we say this installation is x years old, so must be switched off until updated, even cars made in the 19th century are still permitted to travel on public roads.

I am not sure when 50 volt AC was decided as the limit for live items you can touch? With salt on road vehicles you can still get a tingle with 24 volt DC and I know I wore gloves working on fork lift batteries.

My daughter as a very young child was found licking the terminals of a PP3 battery. And we have stories of tree huggers who have found a tree that has touched some power line and claim it is a special tree.

I know the old ELCB-v was set to trip at 50 volt, but I personally think that is rather high, it is rare to find 10 volt never mind 50. All formulas already given, but some times you have to use some common sense, even if the earth rod will work a RCD at 1kΩ, we know that is unreliable, think book says 300Ω but personally would not accept over 60Ω.

 

[JohnW2]

I am not sure when 50 volt AC was decided as the limit for live items you can touch? With salt on road vehicles you can still get a tingle with 24 volt DC ...

... or even less. It's obviously arbitrary but, in terms of most things (the calculations we're talking about, the definition of ELV etc.) it was decided that 50V AC was a reasonable dividing line below which the chance of a serious/fatal shock was very low (and I'm personally inclined to agree that that is a reasonable threshold) , and that sort of thinking seems to persist today. For example, the HSE currently say things like...

... A voltage as low as 50 volts applied between two parts of the human body causes a current to flow that can block the electrical signals between the brain and the muscles....

My daughter as a very young child was found licking the terminals of a PP3 battery.

.. and even with a 1.5V battery - but, to be fair, the unpleasantness of that experience is at least as much to do with electrolysis of saliva as to 'electric shock'.

I know the old ELCB-v was set to trip at 50 volt, but I personally think that is rather high, it is rare to find 10 volt never mind 50. All formulas already given, but some times you have to use some common sense, even if the earth rod will work a RCD at 1kΩ, we know that is unreliable, think book says 300Ω but personally would not accept over 60Ω.

You're presumably talking about RCD tripping as a result of an L-CPC fault, rather than a current through a person. With a 1kΩ rod resistance, the RCD would trip with a CPC voltage of 30V, but I'm not sure how that is likely to arise in practice - the nature of 'faults' is that it's likely to be '230V or nothing'. In the case of a current going through a person from L to CPC, then any rod resistance less that 6kΩ-7k (7,667Ω minus body resistance) will facilitate an RCD trip.

Earth electrodes more than about 200Ω are regarded (I think by BS7671, as well as everyone else) as undesirable, but not so much because of the actual resistance, but more because it could be an indicatioin of an unreliable path to earth (which could get much worse under, say, some weather conditions). Mine is generally around 50Ω, but can be as low as 30-40Ω or as high as ~80Ω under different soil (wewather) conditions.

Kind Regards, John

 

[studentspark]

Thanks for all your time and the replies, Ive given it some thought...
But I totally understand if you want to give this one up as a lost cause.

I am getting myself ties up in knots...

Tv5

• studentspark
• 10 Feb 2020

So

If, for convenience, we take R1 as 1Ω, then, with that ratio, R2 is 1.67Ω.

Well I took that example, and put in numbers that would satisfy Zs requirements
Image 1.

I get a touch voltage of a compliant (Zs) circuit at 117v
So at this point 117v would appear

In figure 3
I have the chap between and Exposed and extraneous CP

The Exposed CP is 117v
The extraneous CP is at what potential ?? around 117v

Touch voltage tells us that a resistance between the two Conductive parts has to be <0.31Ω

Now you have two conductive part R2 resistances of 0.56Ω and 0.05Ω

I presume these resistances in Parallel which would make it 0.04Ω ( if my dodgy maths is correct)

But the chap is in series with the two resistances.

And now I am struggling to know what to work out....
with these numbers what is the touch voltage, between the two CP's

I get mixed up between the high resistance as in 23000 ohms in an extraneous CP to earth , and the low resistance to clear the fault as quick as possible in and exposed CP.
I understand that a high resistance means its not earthy and can't introduce a potential.
I understand the low resistance to create a low impedance path for a fault to clear quickly.
It the 50 v limiting factor.
Its has been said in this post that it can and does rise above 50 v.

Apologies in advance...

 

[JohnW2]

Thanks for all your time and the replies, Ive given it some thought...
But I totally understand if you want to give this one up as a lost cause. I am getting myself ties up in knots...

Hi. Although it says that you posted this at 19:57 yesterday, it has only appeared for me in the last few minutes! I just wanted to let you know that I'm not ignoring you, and will try to help you with your knots shortly!

Kind Regards, John

 

[JohnW2]

Thanks for all your time and the replies, Ive given it some thought...
But I totally understand if you want to give this one up as a lost cause. I am getting myself ties up in knots... Well I took that example, and put in numbers that would satisfy Zs requirements ...

For a start, I'm not sure where your starting figures come from. The maximum Zs for a B32 is 1.37Ω and, per BS7671, 1.5mm² cable is 14.5 mΩ/m (at 70°) and 2.5mm² cable is 9 mΩ/m (at 70°). However, that doesn't alter the concept of your calculations.

You have also confused things considerably by drawing R1 and R2 'the wrong way around' - in terms of standard convention, R2 should be the resistance of the CPC - the one which joins to Ze (at the MET), but you have called it R1 in your drawing.

More importantly, your Image 1 does not represent the actual situation, since the entire current loop includes the impedance of the supply L as wellas Ze - and the voltage driving current around that loop will be the voltage at the transformer, usually more than 230V.

However, you're making things much more complicated that you need. We're only interested about what happens within the installation, so we're not interested in Ze or the external "ZL". You only need to think about the supply voltage at the origin of the installation (say 230V) and the voltages across R1 and R2 (which are in series across the 230V) will depend only on the relative values of R1 and R2. Doing it by Ohm's Law ...

If you have 230V across R1+R2, then the current (through both), say I, is 230/(R1+R2)
The voltage across R1 is then (I x R1) and the voltage across (R2 is I x R2)

Because you have drawn R1 as being the CPC resistance (and R2 as being the L resistance), then, very confusingly, it will be the junction between your R1 (which, conventionally, should be R2) and Re in your Image 1 will be the MET. An extraneous-c-p, should be bonded to the MET and, since no/negligible current is usually flowing through that bonding cable (hence no VD along it), the extraneous-c-p will be at the potential of the MET. Therefore, a person touching the 'live' exposed-c-p (at the junction between R1 and R2, where the 'short' has occurred) and an extraneous-c-p will experience a voltage across them equal to the voltage across your R1 (which is what we would normally call R2).

Even when the person is touching the two things, the current flowing through the bonding conductor (to the MET) will only be the current flowing through the person ('negligible' in terms of the big picture). There will therefore be no significant VD in the MET, so the potential of the extraneous-c-p will still remain at roughly MET potential - hence pd across the person will remain as roughly the voltage across your R1 (i.e. conventional R2!).

Exactly the same would be true if the person simultaneously touch the 'live' exposed-c-p and the exposed-c-p of something else, which had a separate CPC path back to the MET. This second ('non-faulty') exposed-c-p would then be exactly the same as the extraneous-c-p discussed above.

As for the "50V", as I've said before, in the situations we are discussing, in the absence of 'additional measures' nothing can stop the pd between two exposed-c-ps, or between an exposed-c-p and an extraneous-c-p rising above 50V under certain fault conditions - as above, the voltage between the live exposed-c-p and something connected to the MET (a bonded extraneous-c-p or an exposed-c-p connected to MET via a different CPC) will be that voltage across (conventional) R2 (your R1) that you can calculate by Ohms Law. What id required is that a protective device rapidly clears the fault (i.e. limits its duration) IF that voltage rises to above 50V

Those 'additional measures' are what Supplementary Bonding is all about. If you join together all the 'simultaneously touchable' exposed- and extraneous-c-ps (with local bits of G/Y cable), then you will have minimised the potential difference (which is what I think you mean by ;touch voltage) that can ever exist between any two of them.

Does any of that help at all?

Kind Regards, John

 

[studentspark]

Hi. Although it says that you posted this at 19:57 yesterday, it has only appeared for me in the last few minutes! I just wanted to let you know that I'm not ignoring you, and will try to help you with your knots shortly!

Thank you very much.
I posted it last night, but it was waiting for moderation. I presume they like to check it when an image is attached?

Im reading through your post now. Thanks again