92 volt touch potential

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mbrooke

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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?


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? o_O o_O


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GoldDigger

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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?




Huh? o_O o_O


View attachment 2552908
Looking at the diagram, they appear to be calculating the voltage divider effect using the resistance of a combination EGC and other parallel ground paths that has a lower resistance than the path of the ungrounded wire. If the two resistances were equal, the the voltage would be 115 or so. Basicallly 1/2 of the line to neutral voltage.
 

mivey

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It is just a standards calculation generic assumption that the source will maintain at least 80% nominal voltage. Thus 115 becomes 92. The 0.4 sec clear is based on certain generic assumptions which may not hold true for a specific install.

The actual touch voltage can be higher of course depending on source stiffness and relative size of phase conductor to neutral conductor, etc.
 

mbrooke

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It is just a standards calculation generic assumption that the source will maintain at least 80% nominal voltage. Thus 115 becomes 92. The 0.4 sec clear is based on certain generic assumptions which may not hold true for a specific install.

The actual touch voltage can be higher of course depending on source stiffness and relative size of phase conductor to neutral conductor, etc.


Ahhh! I needed your knowledge. :)

Alright, dumb this down for me... a short circuit will pull the voltage down at the spades of a transformer due to its internal impedance (R+jX) of said transformer, thus the source (X0,X1) is not actually 230 volts but rather 184 volts and as such the restive divider will give 92 volts relative to remote earth assuming phase and EGC are the same size?

Is this why circuits over 32 amps allow for a 5 second disconnection time? Because larger circuits will pull down the voltage more? And lower voltage means lower touch potential?
 

mbrooke

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Looking at the diagram, they appear to be calculating the voltage divider effect using the resistance of a combination EGC and other parallel ground paths that has a lower resistance than the path of the ungrounded wire. If the two resistances were equal, the the voltage would be 115 or so. Basicallly 1/2 of the line to neutral voltage.


Ah, good point. They might be taking other paths into account. Perhaps even local or building elevations in voltage?
 

don_resqcapt19

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The touch voltage is equal to the voltage drop on the EGC. The amount of fault current flow combined with the impedance of the fault return path results in the touch voltage.
 

mivey

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thus the source (X0,X1) is not actually 230 volts but rather 184 volts and as such the restive divider will give 92 volts relative to remote earth assuming phase and EGC are the same size?
exactly.
Is this why circuits over 32 amps allow for a 5 second disconnection time? Because larger circuits will pull down the voltage more? And lower voltage means lower touch potential?
I would have to read the 32 amp / 5 sec context. Sorry I haven't read it but am busy today.

There is a volt drop limit before we increase supply capability and/or feeder size. After all, we want the breaker or fuse to clear properly and our equipment to function properly so we want to be able to push enough current.. 20% is an extreme voltage drop that you might normally only see on things like motor starts.
 

mbrooke

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Just a number with no science behind it. You can not get a valid number without knowing the fault current and a lot more things.


Good point, but it seems like the 0.4 second disconnection time is based on a 92 volt touch potential if I'm reading the body graph right. So the code making process has to have agreed that the typical properties of an electrical installation are found not to subject users to over 92 volts during a fault.
 

mbrooke

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Electricity gets more fascinating the more you think about it. To cool :cool:

I would have to read the 32 amp / 5 sec context. Sorry I haven't read it but am busy today.

Table 41.1:

1594540024223.png




There is a volt drop limit before we increase supply capability and/or feeder size. After all, we want the breaker or fuse to clear properly and our equipment to function properly so we want to be able to push enough current.. 20% is an extreme voltage drop that you might normally only see on things like motor starts.


Outside of the NEC right- however- voltage drop and fast clearing are not always inclusive or accompanying one another. You can still be within VD limits yet not trip a breaker for a Line to Ground fault. This is actually most prevalent in circuits under 600 volts.

Using an example you are familiar with think of distribution lines which require a far end down stream re-closer. Customers are within 5% of voltage limits however a fault will not trip the substation breaker even on 51N or 51G. (51G if you don't wire the neutral through the IN terminals if I'm remembering it right)
 

Julius Right

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If the rated voltage drop at rated current is 3%[for a feeder] and the fault current is 6.7*Irated the source voltage drop is 20%.​
 

paulengr

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Those trip times are crazy and basically incorrect. To begin with 0.04 seconds would mandate a 1.5 cycle breaker with a 1 second trip unit. That’s realistic for breakers under about 200 A but completely unrealistic above that. A vacuum breaker for instance is a minimum 2 cycles for the fastest ones with magnetic trip units. Only smart fuse systems can trip fast enough. Second it completely ignores coordination which required at least one additional cycle even for zone selective relaying.

Going the other way at 92 V human body resistance is around 1 Kohm whether we follow IEEE or IEC standards. Thus the current is 92/1000 or 92 mA. That is just below the minimum threshold for fibrillation to occur (100 mA).
 

don_resqcapt19

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Good point, but it seems like the 0.4 second disconnection time is based on a 92 volt touch potential if I'm reading the body graph right. So the code making process has to have agreed that the typical properties of an electrical installation are found not to subject users to over 92 volts during a fault.
There is no way that the touch potential under fault conditions can be limited to 92 volts for all circuits. I believe the effects on the body are based on current flow through the body and not a specific voltage level. Of course the voltage combined with the impedance of the path determines the voltage.
 

GoldDigger

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There is no way that the touch potential under fault conditions can be limited to 92 volts for all circuits. I believe the effects on the body are based on current flow through the body and not a specific voltage level. Of course the voltage combined with the impedance of the path determines the voltage.
And there is a non-linear component to the skin resistance, to the extent that the outer layer of skin has a high resistance until punctured by a high enough voltage, at which point the current increases dramatically.
 

mbrooke

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Those trip times are crazy and basically incorrect.

How so? They are correct in terms of the maximum amount of time the human body can be subjected to a given circuit's touch potential as derived from IEC60479-1.

To begin with 0.04 seconds would mandate a 1.5 cycle breaker with a 1 second trip unit. That’s realistic for breakers under about 200 A but completely unrealistic above that.A vacuum breaker for instance is a minimum 2 cycles for the fastest ones with magnetic trip units. Only smart fuse systems can trip fast enough. Second it completely ignores coordination which required at least one additional cycle even for zone selective relaying.

Right- Table 41.1 applies only to circuits 32 amps and under. Above 32 amps you are permitted to have a 5 second disconnection time for a TN system and up to 1 second for a TT system.


1594633358511.png


Going the other way at 92 V human body resistance is around 1 Kohm whether we follow IEEE or IEC standards. Thus the current is 92/1000 or 92 mA. That is just below the minimum threshold for fibrillation to occur (100 mA).

Which is you want to trip an OCPD promptly during a fault.
 

mbrooke

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There is no way that the touch potential under fault conditions can be limited to 92 volts for all circuits. I believe the effects on the body are based on current flow through the body and not a specific voltage level. Of course the voltage combined with the impedance of the path determines the voltage.


Higher voltages mean more current flow through the body for a given resistance according to ohms law. Hence why Table 41.1 requires successively faster disconnection times as the nominal steady state line to ground voltage increases.

Here is the thing. I'll agree with you- it is theoretically possible to have more than 92 volts relative to remote earth in a scenario where the EGC is smaller than the phase conductor(s). This can happen if the EGC is sized based on the adiabatic method.
 
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