92 volt touch potential

[Cookie]

I'm really confused by this engineering article. How can the maximum touch voltage be 92 volts on a 230 volt system when in theory it ought to be 115 volts or higher?

TN Systems

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www.gt-engineering.it

In case of a system voltage of 230 Vac phase to neutral, the reason why a time of 0,4 seconds is specified is because 0,4 seconds is the maximum time a person can be subject to 92 Vac. That is the normative touching voltage in a TN system operating at 230 / 400 Vac.

Huh?

 

[pc1966]

Very strange, it appears to be a fuse's current/time curve but a "TN" system is so generic that you can't equate a given current (thus clearing time) to a specific touch potential.

 

[Megawatt]

I'm really confused by this engineering article. How can the maximum touch voltage be 92 volts on a 230 volt system when in theory it ought to be 115 volts or higher?

TN Systems

...

www.gt-engineering.it

Huh?

Cookie it’s the neutral not the 120vac. If you lift the neutral while the corresponding breaker is on that’s where you will get bit by 92vac

 

[pc1966]

Thinking about it, perhaps the curve they show is the shock risk based on a nominal 2k or whatever skin impedance. I.e. that the limit is 92V for 0.4s

However, that is a rather odd way to present it, as normally the shock risk is better represented by the current (as in the RCD trip characteristics and shock risk).

 

[Lucien Nunes]

Touch voltage on a TN system is sometimes defined as 0.8 Uo/2 if Rphase = Rpe, giving 92V for 230V systems. But I can't remember for toffee apples where the 0.8 comes from!

it’s the neutral not the 120vac

This is specifically on 400/230V wye systems as used in Europe.

 

[Cookie]

Cookie it’s the neutral not the 120vac. If you lift the neutral while the corresponding breaker is on that’s where you will get bit by 92vac

Right, but if you touch hot to ground on a 120 volt circuit, 60 volts will appear at the fault point relative to remote earth before the breaker opens.

Similarly 115 volts would appear to remote earth during a fault.

I can't see where 92 volts comes from.

 

[Cookie]

Touch voltage on a TN system is sometimes defined as 0.8 Uo/2 if Rphase = Rpe, giving 92V for 230V systems. But I can't remember for toffee apples where the 0.8 comes from!


This is specifically on 400/230V wye systems as used in Europe.

What is Uo again?

I'd be curious as well on 0.8.

I have a theory that simply shorting out a circuit drops the initial voltage at the spades of a transformer.

 

[Megawatt]

Right, but if you touch hot to ground on a 120 volt circuit, 60 volts will appear at the fault point relative to remote earth before the breaker opens.

Similarly 115 volts would appear to remote earth during a fault.

I can't see where 92 volts comes from.

I’ll tell you what take a 120vac live wire to ground and see what happens. I don’t see how you can measure voltage by doing that. PS wear protective face shield and glasses

 

[Cookie]

I’ll tell you what take a 120vac live wire to ground and see what happens. I don’t see how you can measure voltage by doing that. PS wear protective face shield and glasses

Oscillioscope or if the breaker clearing time is long enough a meter set to its max peak volts setting.

One lead to the EGC at the fault point another lead to an isolated ground rod.

1000 ohm resistor in series if you want to make it more realistic.

 

[pc1966]

I can't see where 92 volts comes from.

I think it is very misleading.

I think what they are saying is 92V is the highest acceptable touch potential at 0.4s exposure time. From that you would look at your OCPD current for 0.4s disconnection to set the Ze+R2 upper limit (but they don't say that).

 

[Cookie]

I think it is very misleading.

I think what they are saying is 92V is the highest acceptable touch potential at 0.4s exposure time. From that you would look at your OCPD current for 0.4s disconnection to set the Ze+R2 upper limit (but they don't say that).

So how is 0.4 seconds allowed on a 230 volt circuit? 115 volts would be present during a fault.

 

[Cookie]

Thinking about this- is it possible they are taking Ze into account with bonding inside the building to water pipes, rebar, ect?

 

[Cookie]

Alright- I have a possible answer. I spoke with a seasoned EE. He thinks it has to do with the fact the voltage at the spades of the transformer will drop below 230 volts during the fault. A 32 amp circuit less likely to pull it down considering the run and wire size involved.