touch voltage

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S

sparkyork

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when i did my apprentaship for a pub refurbing company i was always tripping the rcd when cutting through N - E. is this tripping because of touch voltage in the earth?

my boss who taught me the little i know some years back told me that it was cos of some juice in the neutral. but given how an rcd works it must be juice in the earth?

if main equipotential bonding is not up to scratch, can this effect this "touch voltage" or other types of bonding.

where does this voltage come from, is it form emf's etc? induced voltage and all that?

just very curious about all this type of thing, i dont fully understand it and would like too!!!

wondering if diabetic equations come into it anywhere! lol

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T

tony.towa

RCDs monitor the difference in current between the live and neutral. A variation between the two conductors. By chopping the earth and neutral at the same time, or touching them together the RCD will react to the imbalance and trip. This will also happen if the mcb for the relevant phase is switched off. A pound for every time I've done the same and I could give up work!!!!

S

sparkyork

cheers tony, yeah be a millionaire by now!!

so this is caused by voltage in the neutral then, that momentarily escapes down the earth when the two are shorted together.

wheres this touch voltage come from as well, been thinking bout tt job i went on, with ra of 543 ohms giving touch voltages.

B

Bright-spark

If you isolate a circuit by means of the circuit breaker you are isolating the line conductor of that circuit but the neutral and cpc are still commoned with the other circuits in the db. Therefore when you cut the isolated cable you will in effect short the neutral and earth of the other circuits creating an imbalance in the rcd thus tripping it.

Nothing to do with touch voltage; thats a whole different subject.

S

sparkyork

right i was just associating the 2 as i couldnt rcd test because of touch voltages.

can we talk about touch voltages then please, starting from the very beginnning of time

B

Bright-spark

Touch voltages refer to the difference in potential between two points under fault conditions.

When a fault occurs, current passes along the fault path ie the cpc. As the cpc has a resistance there will be a volt drop along that path. If two points are 'earthed' seperatly back to a common point, say the MET, then there will be a resistance between these two points, even if it is small and therefore there will be a diffence in voltage. If you can touch can these two points at the same time, this is touch voltage.

Touch voltage is proportional to resitance between these points. The higher the resistance the higher the touch voltage. Protective bonding reduces this touch voltage by reducing the resistance between two points ie exposed and extraneous conductive parts.

If you could reduce the resistance to zero then the touch voltage would be zero. That is the idea of supplementary bonding. (Now going out of fashion as other means of shock protection being employed ie rcd's).

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S

sparkyork

cheers brightspark,

so this also why it is very important to have main equipotential bonding installed within an installation. i had a TT with very high elcetrode resistance and no water bond which was giving me touch voltage probs on the test side of things.
whatas the equation for it again 50 divided by rated current of rcd ??

B

Bright-spark

cheers brightspark,

so this also why it is very important to have main equipotential bonding installed within an installation. i had a TT with very high elcetrode resistance and no water bond which was giving me touch voltage probs on the test side of things.
whatas the equation for it again 50 divided by rated current of rcd ??
50 divided by rcd rating is the theoretical maximum resistance for an earth electrode to the ground which for a 30mA rcd is 1667 ohms. Turning this on its head would mean that if you had an electrode to ground resistance of 1667 ohms and a current of 30mA flowed, the touch voltage between the electrode and the ground is 50V. (Maximum electrode resistance recommended in BS7671 200 ohms.)

If you don't have protective bonding on an installation with extraneous conductive parts your disconnection times (determined by Zs protective device combination) are irrelevent and would also result in increased touch voltages.

Touch voltage under fault conditions is determined by actual fault current rather than rcd rating.

S

Shakey

50 divided by rcd rating is the theoretical maximum resistance for an earth electrode to the ground which for a 30mA rcd is 1667 ohms. Turning this on its head would mean that if you had an electrode to ground resistance of 1667 ohms and a current of 30mA flowed, the touch voltage between the electrode and the ground is 50V. (Maximum electrode resistance recommended in BS7671 200 ohms.)

If you don't have protective bonding on an installation with extraneous conductive parts your disconnection times (determined by Zs protective device combination) are irrelevent and would also result in increased touch voltages.

Touch voltage under fault conditions is determined by actual fault current rather than rcd rating.
Hi,

just a bit curious about the last but one bit, why would your disconnection times be irrelevent if there is no protective bonding to the extraneous? (PS, I am not suggesting that protective bonding is not required!)

Yes your protective bonding will contribute to your disconnection times, resistors in parallel etc etc, but lets say you have a TN-C-S system (for example), and the protective bonding was O/C to incoming gas/water etc (again for example), and lets say you had a fault direct to earth (nail through a cable) , and the Zs at that point was, oh lets say, 0.5 ohms, then you would have a fault current of 460 amps, which is well above the 160 amps that a 32A type b (again for example) needs to trip off in the required time

So i am just a bit curious as to why your disconnection times would be irelevent? Yes i agree that the touch voltages could (potentially) be higher (but still not neccesarily dangerous), and it would theoretically raise the disconnection time (although it should still comply), but this a by- product rather than a reason

The protective bonding is not meant to provide a path for fault current, so I cant see how it is connected to disconnection times

good discussion though!

B

Bright-spark

Hi,

just a bit curious about the last but one bit, why would your disconnection times be irrelevent if there is no protective bonding to the extraneous? (PS, I am not suggesting that protective bonding is not required!)

Yes your protective bonding will contribute to your disconnection times, resistors in parallel etc etc, but lets say you have a TN-C-S system (for example), and the protective bonding was O/C to incoming gas/water etc (again for example), and lets say you had a fault direct to earth (nail through a cable) , and the Zs at that point was, oh lets say, 0.5 ohms, then you would have a fault current of 460 amps, which is well above the 160 amps that a 32A type b (again for example) needs to trip off in the required time

So i am just a bit curious as to why your disconnection times would be irelevent? Yes i agree that the touch voltages could (potentially) be higher (but still not neccesarily dangerous), and it would theoretically raise the disconnection time (although it should still comply), but this a by- product rather than a reason

The protective bonding is not meant to provide a path for fault current, so I cant see how it is connected to disconnection times

good discussion though!
Sorry didn't make myself clear.

Electric shock effects are a combination of current flowing through the body and the length of time it flows for.

The length of time it flows for is determined by the Zs and protective device combination.

The amount of current flowing through the body is determined by body resistance and the voltage (touch) driving it. Main protective bonding reduces touch voltage thus reducing shock current through the body. Leaving out main protective bonding increase touch voltage, thus increasing body current and thus shock risk.

Supplementary bonding reduces touch voltage even further thus lowering risk. So why have we gone away from supplementary bonding in the 17th?
This is because we have in effect reduced the disconnection time by the use of rcd's for bathroom (lowered body resistance area) circuits.

The point I was trying to make is that shock protection relies on both disconnection times and protective bonding one can't acheive the desired result with out the other hence why a failure of either is classed as a '1' on a Periodic Inspection Report.

Hope this makes sense.

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