Rcd Test Problem

[Wattsy]

Hi all,

I was doing a pre work survey on a static home today where the owner want a couple of circuits extended.

Its a TT and the Ze was 320ohms which i know is higher than recomended.

Its got an old Wylex board supplied through a 30ma RCCB switch.

I tryed to do and rcd test with my Megger 1553 and it kept coming up with >50v. Which i understand is the touch voltage of the cpc. Can anyone tell me why this is the case.

Any help would be appreciated

Many thanks

 

[sparkytim]

Which of the RCD tests did this show up the >50 volt?

 

[septiclecky]

Is it when you connect the probes?

 

[NozSpark]

Have you done an earth loop L-N.

Justa a thought,, if you have a high L resistance combined with a high E, then your touch voltage could be a bit high.. If all tests are OK though and it still won't RCD test, then do the RCD test across Outgoing L and incoming N on the RCD

 

[m4tty]

Have you done an earth loop L-N.Justa a thought,, if you have a high L resistance combined with a high E, then your touch voltage could be a bit high.. If all tests are OK though and it still won't RCD test, then do the RCD test across Outgoing L and incoming N on the RCD

Obviously spot on advice as always Noz but why test across outgoing L and incoming N. You obviously know why but im trying to understand why you do that. Can you explain it to me.

Thanks

Guinness

 

[NozSpark]

It will test the RCD,, just like the test button, but with readings that mean something

and is safer that incomming L and outgoing N as your N probe will always be "live" until you remove the L probe.

 

[wozz]

doesnt that mean less than 50 volts, as in not energized?

 

[m4tty]

yeah think wozz has got it. If the meters not reading any voltage thats what it says i think.

 

[NozSpark]

No.

My guess is he has a Megger MFT... and it means that the touch voltage would be too high

 

[septiclecky]

doesnt that mean less than 50 volts, as in not energized?

> means greater than

< means less than

Therefore with him saying >50 there is more than 50 volts

 

[ADS]

The Megger MFT has two touch voltage settings- 25v and 50v.

It can be set to display which setting you have it on during the test.

Are you sure that's not what it is doing, rather than displaying a fault

 

[Wattsy]

Hi, I used the socket on my MFT 1553 on the auto setting, it starts the

test then says the touch voltage is >50v. Could this be because the Ze is 321ohms? If so measuring it at the Rcd out L - in N as you said should work but is it acceptable?

Thanks again

 

[NozSpark]

IIRC not withstanding what I have said previously, you could have a N-E fault on another circuit

N-E faults do not always trip RCD's in TT installs

 

[sparkytim]

Yes you can test RCD where you like, on TT it won't make a lot of difference

The >50 v indicator is warning that the Fault touch voltage would be in excess of 50 Volts so it won't allow you to continue, not sure why at 321oms as 1666.66 ohms would be limit for that at 30 m A. Maybe its preprogrammed to give that out at anything above 200 ohms RECOMMENDED in BRB Pg 50.

If you can reduce the Ze by installing additional Earth stakes then may be You would be able to do the tests.

 

[JUD]

The >50 v indicator is warning that the Fault touch voltage would be in excess of 50 Volts so it won't allow you to continue, not sure why at 321oms as 1666.66 ohms would be limit for that at 30 m A. Maybe its preprogrammed to give that out at anything above 200 ohms RECOMMENDED in BRB Pg 50.

Unless he has it set on 300mA by mistake. The limit would then be 167ohms would it not. :C

 

[Wattsy]

It's definately set at 30mA.

Nozspark, the N - E fault would make sense but when i tested the PFC the PSSC was 1.09kA and the PEFC was showed <1A so would this be the case if the E & N were connected together?

These are all the tests i have done as i didn't have much time at the end of the day yesterday. I will be going back on monday though so if anyone can suggest any more tests i'd be very gratefull.

Its a ring final circuit

Ze = 320ohms

ZS (direct from socket on circuit to be extended) = 321ohms

r1 = 0.63ohms r2 = 0.78ohms rn = 0.62ohms (r2 seems a bit low?)

PSSC = 1.09KA

PEFC = <1A

Calculated cable length = 85m

RCD test from socket

= No result (>50v)

 

[matt.leung]

have u done a full polarity check at the incoming supply.... check between L-N, N-E, E-L. if polarity is not correct the >50v can sometimes come up on the meter... L-N could be reveresed...

 

[ADS]

It's definately set at 30mA. Nozspark, the N - E fault would make sense but when i tested the PFC the PSSC was 1.09kA and the PEFC was showed <1A so would this be the case if the E & N were connected together?

These are all the tests i have done as i didn't have much time at the end of the day yesterday. I will be going back on monday though so if anyone can suggest any more tests i'd be very gratefull.

Its a ring final circuit

Ze = 320ohms

ZS (direct from socket on circuit to be extended) = 321ohms

r1 = 0.63ohms r2 = 0.78ohms rn = 0.62ohms (r2 seems a bit low?)

PSSC = 1.09KA

PEFC = <1A

Calculated cable length = 85m

RCD test from socket

= No result (>50v)

Hi mate,

Those readings look ok to me.

Your PEFC will be less than 1A with a Ze of 320 ohms - 230/320= 0.72A

This is why RCD is required for for earth fault protection

RCD test from socket

= No result (>50v)

If I'm reading this bit correctly then you shouldn't get a trip result at half X

I've just re-read my instructions and the Megger gives a touch voltage reading on a bar graph (if selected).

You can set the touch voltage limit for RCD testing at either 25v or 50v and it will display the reading with the trip time.

The only time the Megger will display >50v is if you try to perform a 'dead test' on a live circuit.

.

.

.

.

Edited for accuracy

 

[m4tty]

Are you sure the megger switch has selected the correct test as sometimes (i read) that when you twist the dial it can sometimes not select perfectly. It could have stuck on a dead test when you thought it was on a live test therefore showing the <50v symbol.

Just a thought

 

[ADS]

Hi again,

I've just ran some RCD tests with my Megger.

The touch voltage is shown on a bar graph above the trip time - on each of my tests the graph didn't move, but I am testing on TN-C-S.

I must admit, I've never had a case where the set touch voltage (25v or 50v) has been exceeded on my megger, so I can't say for sure what it would display if this were to happen.

As someone mentioned earlier, though, I can't see how the touch voltage can be exceeded with a Ze of 320 ohms when the calculation for a 50v touch voltage gives us 1667 ohms at 30ma

Bit of a puzzler:C