Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise please

tonycoysh · Dec 3, 2011

Hi, I've done some work for a customer and am testing the circuit for the MWEIC. When doing the Ze the 30mA supply RCD is tripping leaving me with no result.

A little about the installation: The property is in a group of 12 or so properties that are holiday lets. All of the properties are fed from a switch room in the site Laundry building with a 30mA RCD protecting each installation located in a meter box on the side of each building along with a meter. All of the properties have TT earthing arrangments. Now I can get a Ze (105 Ohms) result if I test on the meter side of the RCD (RCD 'off' and earth disconnected from Henley block) but when I try testing from the CU (installation isolated and Earth disconnected) I just trip the RCD.

My question has 2 parts,

1: does anyone know a way around this?
2: Is it acceptable to do an R1+R2 on the feed + earth from the RCD to the CU and add that to the Ze I have. Record that result as the Ze and make a note on the MWEIC. I would then have to do the actual R1+R2's and add it to the overall Ze as I expect I will get a trip doing Zs tests as well.

I haven't gotten as far as doing any tests on the RCD I will do them when I return to the property this week.

Thanks in advanced

spark 68 · Dec 3, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

You only have one Ze, and that is at the origin, if you are talking about the 'incoming Earth' at a downstream DB, then this is marked as ZdbXX (X= the number or name of the sub DB), this is in reality a Zs, comprised of THE Ze + the (R1+R2) of the submain, and then add the (R1+R2) of the final circuits of the sub DB for the Zs of your finals at this DB..

If your submain is RCD protected (in this case), you can either use the true Ze(at the origin) + (R1+R2) of the submain (the supply cable) method, or use the No-Trip test.

I hope this helps.

Deleted member 9648 · Dec 3, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

Top reply spark 68...:yes:

topquark · Dec 3, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

wirepuller said:
Top reply spark 68...:yes:

Comprehensive!

tonycoysh · Dec 4, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

Thanks for the excellent reply Spark 68. It has helped me get my head around the setup a bit more as far as recording the tests etc. The submain isn't RCD protected, the RCD is sited on the side of the property a 100 or so meters away from the switch room. This is the RCD I am tripping out even on 'no trip'. So I appreciate my Ze is point of origin (switch room) and my ZdbXX would be the Earth Loop from the db of the property through the TT arrangement back to the switch room, only like I say I am tripping the RCD. So... going back partly to my original question, as I can get a reading from the 'live' side of the RCD and from the earth (RCD 'off' and earth disconnected from the Henly block) is it then acceptable to do an R1+R2 equivalent reading on the remainder of the tails from the RCD and Henly block to the db, add that to my original reading and record as ZdbXX.

From then on I am fine because I can do R1+R2 on the final circuit. and record it as Zs = ZdbXX + (R1+R2)

My other question arising from your reply is: Would I not be recording the ZdbXX as the Ze of the property? Because the switch room is in effect a DNO supply I am only measuring the Earth loop from the transformer (located where ever it is) through the switch room and the earth through the earth rod. Therefore isn't the Ze of the switch room included in the Ze (or ZdbXX) of the property.

Thanks for all of your help, sorry if some of my terminology is off or I am sounding thick. I am relativly new to testing and am fine in a 'normal' property but this setup and the issue of tripping the RCD under 'no trip' has caused me to seek a little advice. Which is good in a way as I wouldn't have learnt what I have from your response so far.

Thanks

spark 68 · Dec 4, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

Hi mate,

You would record Zdbxx on that SoTR (one schedule for each DB) in place of Ze, This Zdb is then added to the (R1+R2) of all of your finals for that DB.

Record Zdb as your Ze for that DB.

Eg. Ze + (R1" + R2") + (R1 + R2) = Zs for final circuits of that DB.

Eg.2 Ze +(R1" + R2") = Zdb

Ze is at the origin

R1" and R2" is the supply cable to the DB (this is your sub main)

R1 and R2 being the final circuits for that DB

SPARTYKUS · Dec 4, 2011

Re: Ze test with RCD in supply using Megger 1720MFT tripping on No trip, Advise pleas

spark 68 said:
Hi mate,

You would record Zdbxx on that SoTR (one schedule for each DB) in place of Ze, This Zdb is then added to the (R1+R2) of all of your finals for that DB.

Record Zdb as your Ze for that DB.

Eg. Ze + (R1" + R2") + (R1 + R2) = Zs for final circuits of that DB.

Ze is at the origin

R1" and R2" is the supply cable to the DB (this is your sub main)

R1 and R2 being the final circuits for that DB