TOUCH VOLTAGE

[peetee]

Please could someone explain Touch Voltage to me and do I have to check for it when installing socket outlets and how I go about testing for it

 

[Zeespark]

50 Volts AC.

AndyGuinness

 

[ISSWWW]

As Andy says you are checking the posible potential to earth and the result should be less than 50 volts.

Most modern testers check this by means of a touch plate on the tester, the Dilog 9083p for example just does this as a pre test and prevents any testing if the potential is greater than indicated above.

Just as a note to the above a result is not indicated on the tester it just indicates if it is greater (indication is different on each meter).

 

[Robin Spark]

Please could someone explain Touch Voltage to me and do I have to check for it when installing socket outlets and how I go about testing for it

Hi Peetee,

1. Touch voltage is the maximum allowed fault voltage that could appear on exposed metal work if their were a earth fault on that circuit before the protective device operates.

2. Maximum Touch Voltage allowed is now 50volts (used to have 25v on construction sites) as anything above is dangerous to life.

3. The regulations specify 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.

4. 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= resistance

So 333.33 ohms is the maximum Zs (EFLI) that would be possible before a fault voltage of 50v would appear and most modern testers would abort the test so as not to cause a danger to the test operator or anyone else in that building.

Hope this makes sense

Also as high test currents would be needed to check circuit breakers then we dont normally test these, however the manufacturer would do on production.

So what we do is again look in the regs book or onsite guide which will give us the maximum EFLI allowed for a particular circuit breaker or fuse. Then so long as the circuit is installed correctly and EFLI does not exceed the recommended value then we can be sure that the protective device will again disconnect quickly before 50volts is reached on any extraneous/ exposed metal work.

 

[binky]

Found some info years ago about touch volatges, fault currents and bonding and how bonding in conjunction with CPC and mian earth is suppossed to keep touch voltages down to less than 50v on extraneous metal parts until MCB trips. If I remember rightly it was in the section about earthing and bonding. I'll see if I can find it again, unless anyone else knows where it is??

 

[SPECIAL LOCATION]

415 page 59 (red book) [not gone green yet!]

specifically 415.2.2.

where it says that the resistance between two simultaneously accessible exposed conductive parts and extraneous conductive parts shall be

"less than or equal to 50v / the current rating of the protective device."

Which means the voltage difference between two bits you can touch should not be allowed to rise above 50v in A.C. systems.

AKA 50v "touch voltage"

 

[park1]

There is no requirement to limit touch voltage in general installations, only special locations.

 

[Doc Hudson]

There is no requirement to limit touch voltage in general installations, only special locations.

This thread is nearly a year old, I suspect the OP has got his answer by now, also your advice is contradicted by 415 anyway.

Doc H.

 

[Steve3948]

There is no requirement to limit touch voltage in general installations, only special locations.

What as in his post above :^O

 

[park1]

I find it scary that electricians do not!! Understand touch voltage.

Touch voltage obeys ohms law

Isn't 415 required in special locations as I mentioned, I stated general.

 

[Manator]

Touch voltages are for all installations not just special locations. Some locations will be lower than 50v dependant on the special area's protected.

 

[park1]

Show me where outside of special locations a requirement to limit touch voltage, you won't.

Take a ring circuit in a domestic, what reg imposes a limit on touch voltage ?

 

[Manator]

Do I have to? Really?

Where on a ring would you expect a touch voltage ? Why should one regulation only apply to a certain location and not another?

 

[park1]

Yes please, a reg number too.

Thanks.

 

[Manator]

I assisted in writing some of them, which one would you like in particular?

 

[park1]

I assisted in writing some of them, which one would you like in particular?

I'd like you to show me any in general installations, that excludes 415.

 

[Manator]

First of all we can go to the beginning and start at regulation 131.1 which in itself runs into regulation 131.2, and these are only the basic principles, however when you read them you will see a need to do two important things.

---------- Post Auto-Merged at 22:35 ---------- Previous post was made at 22:14 ----------

If I am moving too fast for you and you need extra time, look at regulation 131.2.2 which also highlights your responsibilities.

---------- Post Auto-Merged at 22:36 ---------- Previous post was made at 22:35 ----------

I am now on page 17 of a book with 464 pages.

 

[park1]

I see no limits on touch voltage for general installations?

 

[Manator]

So you make up your own touch limits?

 

[park1]

No, I may design to a certain touch voltage, BS 7671 for general installations, requires disconnection times and bonding, you show me where a limit is placed, look st the circuits within the onsite guide say for a tncs, Ze 0.1, do you think the touch voltage is below 50 v