rcd touch voltages

[iceman]

hi could you give me some advice regarding calculations for RCD touch voltages

30mA RCD
RA X I^n = < 50v
166 x 0.3 = <50v
100mA RCD
RA x I^n = <50000v
500 X 100 = <50000v

300mA RCD
RA X I^n = < 50100v
167 x 300 = <50100v

500 mA RCD
RA X I^n = <50000v
100 x 500 <50000v

any help on this would be great chears

 

[xraytek]

Not sure what advise you want but here goes:

If a 30mA device is on a circuit with a Zs of 10Ohms then the touch voltage will never go above 0.3V before the device trips. You calculate it by using Ohms law.

A 30mA device on a circuit with a Zs of 1670Ohms will have a touch voltage of 50.1 Volts which is why the maximum allowed Zs for a 30mA RCD is 1667 Ohms to ensure the touch voltage does not go above 50V.

Hope that helps

 

[iceman]

when i last did my 2391 exam i had a queston askin how to work out the touch voltage on 500 mA RCD I knw how to work out the max earth loop imperdences as you said in ya last post, but if i dont had to touch voltage i cant workout the earth loop imperdences cheers

 

[xraytek]

With Ohms law provided you have 2 of the values you can easily work out the third.
You know the current as this is the RCD rating, so if you know the voltage you can work out the impedance. Or if you know the impedance you can work out the voltage.

Quite simple when you get down to it but probably made more complicated by the way City & Guilds ask their questions

 

[iceman]

yeah there terrable for that i rememeber one question i had on continuity of a ring final circuits
it was worded somethin like this name all the connections for a rinal final circuit test you still gotta mention the safe isolation procedure, i think the best way to describe tht is try and remember the the three stages told in GN3