92 volt touch potential 2

[Mbrooke]

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?

https://www.gt-engineering.it/en/en-iec-standards/iec-60364-4-41/tn-systems

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?

 

[Mbrooke]

In my opinion, nevertheless the minimum voltage to ground it could be 0.8*115=92 V if the PE connection is damaged and interrupted it could be total Uo=230 V so the disconnecting time remains 0.4 sec.

TT requires 0.2 seconds:

 

[waross]

Ok- but why does the voltage dip to 92 volts and not 115 volts?

First, It's the law, it doesn't have to make sense.
Second, with 230 Volts to ground, you or I could easily achieve that 92 Volts calculated touch voltage on a 230 Volts to ground circuit by suitably increasing the size of the equipment grounding conductor.
You and I need more background information to fully understand the original statement that was probably originally written either in Italian or by a writer speaking English as a second language.
As a statement it may not be valid.
It does make sense if it is a standard to be met.
ie: The equipment grounding conductor shall be sized so that the touch voltage does not exceed 92 Volts on a 230 Volt to ground circuit.
That requirement may exist in documents that are not present in this discussion.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

The thing is the equipment grounding conductor is not sized based on step potential. Its sized based on either the adiabatic method or Table 54.3- the adiabatic method yields very small EGCs.

The Italain manufacturer did not come up with 92 volts- IEC 60364 and IEC 60479-1 did:

https://en.wikipedia.org/wiki/Electrical_injury#/media/File:IEC_TS_60479-1_electric_shock_graph.svg

 

[7anoter4]

BS7671 chpt.411.4.6 Table 41.2 for instance Irated=25 A Zs=1.35 ohm
Ifault=230/1.35=170.37 A[6.8*Irated]
If the rated source voltage drop is 3%[NEC] at fault the source voltage drop will be 3*6.8=20.4%

 

[waross]

Line to line voltage drop is not directly related to line to ground voltage during a fault.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Wait- 7anoter- are you referring to voltage drop at the transformer during a fault?

 

[7anoter4]

Of course: transformer and feeder and what ever is included in “source”.
[230 V it is line to neutral [or grounding] in Europe -according to IEC 60038]

 

[cuky2000]

The 20% phase-to-neutral voltage drop is reflected In the formula below calculating the prospective max.touch voltage as follow:
U_d = 0.80xUo/2 = 0.80x230V/2= 92V. _{[Assuming a resistance of the ground loop: Rph=RPE; Rph/(Rph+RPE)=1/2
Where; Rph= resistance of the phase conductor & RPE Resistance of the neutral Uo=nominal phase-to-ground voltage=230Vac}

The protective device (RCCB=residual-current circuit breaker or GFCI) shall sense and remove the fault current on he neutral Within 0.4sec required by the IEC Std.

 

[Mbrooke]

That 20% have to come from the source, gen or transformer I'd think.

My understanding is that 0.4 applies to the breaker- not the RCD even if one exists. If say the RCD fails, you still want a low Z path to open the breaker.

 

[waross]

Thank you Cuky for providing the background information.
I don't like it, but I don't hold you responsible.
If it's the law, it doesn't have to make sense.
In our jurisdiction, we have some LOTO regulations that were written by idiots, go against hard lessons learned over 50 years ago and are dangerous to life and safety.
It's the law and you can't fight the culture.

That 20% assumes the transformer impedance, (Ohmic, not PU), which is variable and unknown. The source could be a 5 KVA transformer, it could be a 1000 KVA transformer.
That 20% assumes the feeder resistance. (Impedance?) The feeder may be a few feet long, the feeder may be over 100 feet long.
That 20% assumes the branch circuit resistance. The branch circuit may be a few feet long, the branch circuit may be over 100 feet long.
Who on earth would ever size a transformer based on the voltage drop on feeders of unknown length caused by a fault on a branch circuit of unknown length and capacity?
Too many ASS-U-ME tions for me.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@Waross- I think I just might be able to agree with your assessment/interpretation of the situation. I'm doing some detective work on just where/how the IEC obtained that 0.8 multiplier but I'm coming up cold. Even the IEC veterans are having a hard time finding an inference.

Personally, I see no reason why a long enough run from a very large main switchboard won't cause 115 volts (or even higher) potential relative to remote earth. Look into the adiabatic method... the EGC can get really skinny.

FWIW- you can have up to 5 seconds on a circuit over 32 amps when dealing with a TN supply and 1 second disconnection time when dealing with a TT supply.


Then again it is possible that under typical supply conditions with typical circuit runs adhering to table 41.1 a typical 46 volt drop occurs at the MDP... I've heard of stories of electricians shorting 14 gauge wire originating out of an FPE panel with every light in the home dimming to a soft glow.

There is also the fact when the service neutral is bonded to the building's equipment grounding system any voltage drop is essentially "eliminated"- every bonded object lacks Ze voltage potential relative to other bonded objects.

 

[waross]

There is also the fact when the service neutral is bonded to the building's equipment grounding system any voltage drop is essentially "eliminated"- every bonded object lacks Ze voltage potential relative to other bonded objects.

Maybe I misunderstood your intent.
Your statement is true as long as there is no current over the bonding conductors.
When there is current over a conductor there will be an IR voltage drop and the bonded equipment will rise above ground potential by that amount.
I know that you know this, but others reading this thread may have been confused a little.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Its possible the IEC is assuming a 15 volt drop on the neutral between the service and transformer with the occupant inside the building. Just a guess.