RCD Trip times

[gselectrical]

Evening

During a PIR on a domestic property earlier today (TT system, reasonably new install) Ra was found to be 568 ohms. Dual RCD consumer unit. Tested RCD's at 30mA. Both triped within limits. Tested at 150mA, No trip. Is this down to the high Ra / zs?

Cheers

GS

 

[gselectrical]

GsThey should trip

230/568 = 400 mA

Rcd 5 x 30 =150 mA should trip within 40ms for additional protection.

tt Reg 411.5.3

Ra x Idelta n eqaul or less 50v

568 x .03 = 17 volts so ok

230/568= 400 mA so should trip within 40 ms, so .2 and 1 sec disconnection times met also

Regards chris

Thanks Chris. I thaught they should trip as the measured Zs although high is nowhere near the 1667 mark. Its not the first time Ive encountered this problem on a TT. Tried replacing RCD, same happens. Could it be my testers? (Fluke) although they are fine on other earthing systems.

GS

 

[Chr!s]

Hi Gs

It may be your meter?

Are you testing at the rcd or at a point on a final circuit?

Regards chris

 

[Andy™]

EveningDuring a PIR on a domestic property earlier today (TT system, reasonably new install) Ra was found to be 568 ohms. Dual RCD consumer unit. Tested RCD's at 30mA. Both triped within limits. Tested at 150mA, No trip. Is this down to the high Ra / zs?

Cheers

GS

did the RCD fail to trip or the test kit refuse to test? if Zs was 568 ohms, at 150mA current, you will have a touch voltage of 85volt. test kit may have refused to do the test because of this?

 

[gselectrical]

Both, at RCD and end of line.

Tester read >40mS, So I am assuming the RCD failed to trip.

 

[NozSpark]

Have you done IR tests on the final circuits? You could have a N-E short.

 

[gselectrical]

Have you done IR tests on the final circuits? You could have a N-E short.

All IR tests are fine Noz.

GS

 

[Andy™]

have you tested RCD in isolation? its well known that certain loads can stop an RCD functioning

 

[gselectrical]

[quote name='Andy

 

[Andy™]

changed the batteries?

 

[rugbymacc]

could both be faulty ???

Also, have you retested your meter on a different rcd/rcbo to prove your meter ?

 

[gselectrical]

could both be faulty ???Also, have you retested your meter on a different rcd/rcbo to prove your meter ?

Battery's new

Meter works fine on any other earthing arrangement, where the Ze is low. Just wont trip at 5I on TT's.

 

[lilman]

My meter shows +40ms if the touch voltage is more than a set limit - can you remove this preset from your meter?

Also - wet around the rod to retest; just to prove your meter is fine

 

[Andy™]

try testing with earth probe on incoming neutral of RCD. will eliminate the high Zs

 

[PC Electrics]

Try connecting your probes across the line output and neutral input of the RCD (simulates connecting the black to a TNCS earth effectively).

This should enable you to test the RCD is working fine (or not) whilst we continue to scratch our heads about the initial "problem".

 

[gselectrical]

My meter shows +40ms if the touch voltage is more than a set limit - can you remove this preset from your meter?Also - wet around the rod to retest; just to prove your meter is fine

I think this is what is happening then (which was suggested earlier)

How can I carry out the RCD test at 5I if the tester is refusing to do it because of the high touch voltage?

GS

 

[Andy™]

How can I carry out the RCD test at 5I if the tester is refusing to do it because of the high touch voltage?

read post 15 & 16.

 

[a1spark]

My meter shows +40ms if the touch voltage is more than a set limit - can you remove this preset from your meter?Also - wet around the rod to retest; just to prove your meter is fine

Heck ! liman are you trying to get our GS arrested for indecent exposure ? ROTFWL

a1spark.

 

[steptoe]

would agree with the over 50v touch voltage shutting down your meter,

if the RCD is tripping out at 1X then it should be tripping at 5X by logic,

it is a fairly high Ra you have,

you really need to try get it down a bit, couple of hundred ohms hopefully,

how many rods installed, what sort of ground is it?

you can get some crystals to help, or salt water, or the ubiqutious wet around the rod(just dont get seen )

 

[a1spark]

would agree with the over 50v touch voltage shutting down your meter,if the RCD is tripping out at 1X then it should be tripping at 5X by logic,

it is a fairly high Ra you have,

you really need to try get it down a bit, couple of hundred ohms hopefully,

how many rods installed, what sort of ground is it?

you can get some crystals to help, or salt water, or the ubiqutious wet around the rod(just dont get seen )

a1spark