earth loop on rcd

[Farmelectrics]

am i right on saying, i have tested two ring mains on db, they are 32a b type breakers there eli tests came back as 0.60 0.58. these are acceptable values, table 2d osg these circuts are protected by a 30ma rcd does this mean these readings would still be ok to the value of 1667ohms as stated on page 50 bs7671

 

[malcolmsanford]

Your Zs value for RCD protected sockets will be 1667 yes

 

[Farmelectrics]

cheers malc does this also apply to rcbos

 

[malcolmsanford]

Yes..........................

 

[Farmelectrics]

malc thanks for help a tester on site saying this is not true and some of the 32a rcbos are reading 1.6ohms which he has deemed a fail.i have to go enginneer and explain now this might sound daft question is that reading 1.667ohms or one thousand six hundread and sixty seven ohms rather confirm it on forumn rather than making --- of my self just seems a high value

 

[Sparky Ninja]

malc thanks for help a tester on site saying this is not true and some of the 32a rcbos are reading 1.6ohms which he has deemed a fail.i have to go enginneer and explain now this might sound daft question is that reading 1.667ohms or one thousand six hundread and sixty seven ohms rather confirm it on forumn rather than making --- of my self just seems a high value

the latter.

It would depend on what the purpose of disconnection is for.

 

[malcolmsanford]

Then your tester is wrong .............it is one thousand six hundred and sixty seven ohms.

Are you sure the tester is not woried as to why there may be a a high value? Not seeing the installation, but for a socket circuit that does seem high unless it is a very long socket circuit.

Though it is well with the RCD capacity, you may want to check why there is such an high Zs value.

What we have to take into account is how he tested them, was it a measured Zs, or as he done the R1+R2 and calculated it. Sometimes on sockets when doing a LIVE test for Zs the socket outlet you try on can give you a high reading, due to perhaps dirty terminal prongs, often on testing if I got an higher than expected result I would remove and insert the plug a few times and re test.

If I was still getting a high value then you may have to look at perhaps a loose connection somewhere.

Tohugh the RCD will operate it is best to find out why something is not as it should be

 

[ringer]

Malcolm has covered the important points to consider in his post #7.

So you understand where the 1667ohms comes from, using Ohm's Law and looking for a touch voltage of 50V on a 30mA RCD

V=IR
230V=0.030A * Rohms
230/0.030 = R
1667 = R

Any resistance higher than that will result in a touch voltage of greater then 50V when the RCD trips (in theory!)
Any resistance less than that will result in a touch voltage of less than 50V when the RCd trips (in theory!) = Happy days.

 

[Farmelectrics]

ringer understand the above long time since i wae using formulas usually design engineer sorted this out so please let me no were im going wrong were does 1667 come from yhe above sum because we have v = 230 we have r =0.030 so v divided by i should give us r am i right because dividing 230 by 0.030 gives you 7666 were have i gone wrong cheers

 

[Farmelectrics]

miss print i =0.030 not r keypad mistake

 

[topquark]

ringer understand the above long time since i wae using formulas usually design engineer sorted this out so please let me no were im going wrong were does 1667 come from yhe above sum because we have v = 230 we have r =0.030 so v divided by i should give us r am i right because dividing 230 by 0.030 gives you 7666 were have i gone wrong cheers

You're not wrong mate, Ringer has made a slight booboo

It should be 50V/0.030A as that's the maximum touch voltage you want any circuit to have.

 

[Des 56]

The maximum touch voltage is 50 volt, work with that figure

 

[Farmelectrics]

thanks chaps problem solved was begining to think id forgot ohms law happy easter

 

[ringer]

Sorry about that - the theory was right but, as has been pointed out, I put the wrong figure in for volts. Glad to hear you have it sorted now.