Touch Voltage...

[Dylboy]

Hello

Hope all is well.

So another thing i want to clarify and understand better is touch voltage.

My understanding is that all extraneous metal parts when touched simultaneously must not have a diffenrce in potential (voltage) of 50V.

So if there is a PEN fault and all the metal work rises to lets say 230V, then even the bonding needs to rise to 230V so there is no difference between touching the outer casing of the oven to the tap.

Now I have been taught a touch voltage calculation of TV = 2 x I x R...

I have no idea where to apply this and how... as a bonding conductor does not have a CB therefore no current.. and on a normal circuit fed from a CB there are its own calcs to make sure that it trips on fault conditions.

Any clarity on this would be highly appreciated



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[GCE]

When you talk touch voltages it will be in relation to earth , as in the mass of ground we stand on.

A bonding conductor is bonded to earth and you can/will have current flow in an earthing conductor especially on an earth fault - On electrode boilers you are allowed 10% of consumption current .On a 800KW boiler to measure 100 Amps on the earth wire is not unusual - You must still ensure the touch voltage does not go above 50V between the shell of the boiler ( metal) and earth ( reg pasted below 6.16.7.3 )


6.7.1.2
To protect an installation or a live conductor against earth fault
currents,
a) the nominal cross-sectional area of the earth continuity conductor shall
have been determined in accordance with 6.12.1.1(d); or
b) suitable protection shall be installed; and
c) where, for practical reasons, the supply to the installation cannot be
automatically disconnected by an earth fault current of double the rated
current (or higher) of the main protective device, as an alternative, an earth
fault detection and disconnecting device can be installed at the point of
control of the installation. The earth fault detection and disconnecting
device shall be so installed that they operate at a current related to the
earth loop impedance which will limit prospective touch voltages under
short-circuit fault conditions to 25 V for a period not exceeding 5 s.

6.16.7.3
Earth leakage protection shall be provided for the circuit that
supplies an electrode water heater, steam generator or boiler. This protection
shall be set to operate in the event of a leakage current exceeding 10 % of the
current consumed by the appliance under normal conditions of operation. The
characteristics of the circuit and of the earth return path shall be such that the
earth leakage device will operate before the potential between earth and the
shell of the steam generator or boiler exceeds 50 V, including under
short-circuit fault conditions.

[Dylboy]

Thanks GCE

For 6.7.12 I see that as if we have a high Ze or a TT system we supply the installation with an ELU which will trip on earth fault to keep it under 25V, is that correct ?

So then with the geyser bascially and the 10% leakage, the only way I see to protect it is with an ELU.

So then in effect everything (class 1) needs to have ELU protection so that it will trip when current goes along the earth conductor so that there is no touch potential.

So the 20Amp feeding the microwave, steel cover, will need to have ELU protection, which is does as it is a socket outlet, so that if the line or even N touches the casing it will trip the ELU before 50V is reached.

But in same as above but if not on ELU, then the CB will trip when Line goes to Earth as that is then a earth fault and if designed correctly should have enough current to instantaneously trip the CB. But fot this to work for those few seconds it won't have a voltage difference to earth of 50V rather 230V.

So then is the only way to achieve 50V to earth for touch voltage is by ELU? Or do we design the CB to fault trip instantaneously (5 x the cb rating for CBI curve 3) to have the resistance of the CPC to have such a low resistance that when it reaches 50V it has enough current to trip the CB.

I.e 50V/100A =0.5 ohm on the CPC ?
The 100A is 5 x 20A of CB to instantaneously trip.

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[Dylboy]

I believe now that is how table 8.1 is calculated.

Example:

40A CB
5 x 50A = 200A

50V/200 = 0.25 ohm

If we apply a factor for voltage variance of 90 % then 0.25 x 0.90= 0.225 ohm

Rounded to get 0.230 ohm which is what is on the table.

I may be way off but it holds on what I tested now, will try do more tomorrow to test theory

Edit: I see this only working with type 3 curve, any other ones such as CBI orange handle as a trip curve max of 10x... this would mean a much much lower ohm on CPC to trip it at 50V.

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[Alfred M]

The maximum earth fault loop impedance (Ze+R1+R2) allowed for each type of protective to ensure that it disconnects within the given time and to ensure that Touch voltages dont exceed 50volts.

If we utilise the formula Volts = currents * resistance and lets give the scenario that we are testing an 30ma RCD on *5 current (150ma then

50volts= 150/1000 * resistance

50volts= 0.150 * resistance

50volts/0.150= resistance

333.33 ohms

So 333.33 ohms is the maximum Zs (EFLI)that would be possible before a fault voltage of 50v would appear.

[GCE]

For 6.7.12 I see that as if we have a high Ze or a TT system we supply the installation with an ELU which will trip on earth fault to keep it under 25V, is that correct ?

Not exactly an ELU but similar - Would be an Adit unit which is settable , same as used on an electrode Boiler

So then with the geyser basically and the 10% leakage, the only way I see to protect it is with an ELU.

I am talking Electrode Boiler , there is a difference between a Boiler and a Geyser

So then in effect everything (class 1) needs to have ELU protection so that it will trip when current goes along the earth conductor so that there is no touch potential.

Your earthing must be correct then you will not get touch voltages

[Dylboy]

Thanks gents and yes sorry meant electrode boiler.

So in essence good earthing and equpipotential bonding is needed followed by the use of ELU to keep it with-in 50Volts.

If no ELU then having a low resistance on the CPC like on table 8.1 will trip the CB at 5 x on an earth fault.

I will do more research and reading as I am not 100% satisfied haha, I now feel there is far to much from following tables for max length, formulas, more tables, Ze, Zs, max Zs, 50V 5x and 10x CBs, touch voltage etc...



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