High zs on RCD's.

[dlt27]

Hi all. I carried out an EICR yesterday. It was a BG split load consumer unit and when testing I noticed the zs values were coming out high.
I noticed on the 1 rcd there was 0.50 Ohms difference between feed and load side of rcd and on rcd 2 there was 0.60 Ohms difference. Just wondered if anybody else had found this and also does anybody think it needs further investigation. I know the zs doesn't really matter because the rcd Will still trip at 1667 ohms. However just wondered what everybody else thought because I've never come across it before. Thanks

 

[ImpededLoop]

I would contact the manufacturer and ask. Personally I would be concerned, but the manufacturer may have a reason.

Edit: Ignore this and go with the comments below!

 

[somersetsparks]

I have had it before with MCB's, I replaced tthem as I thought it's obviously a high resistance connection, probably on the contacts, losing half an ohm is quite a lot.

 

[Deleted member 26818]

As far as I'm aware, RCDs will cause unusual readings when conducting a variety of tests.

 

[Engineer54]

Exactly as Spin has stated, bridge them out if your that worried. So long as the RCD's test out correctly on RCD testing, you shouldn't have anything to worry about....

 

[ipf]

Very rarely come across an RCD protected circuit with a true loop impedance reading. Agreement with the above comments.

 

[baldsparkies]

Rcd's love to add impedance to true Zs values that in reality don't really exist.
Remember your tester is an electronic device just like the Rcd's.
There like a married couple always contradicting each other.
The tester has a no trip function, the Rcd contributes to this with capacitance back emf's and all those other thingy ma gigs.
If extreme, it may even throw up a noisy circuit fault code on yer tester.
Take it out of circuit and replace with an mcb for test purposes and take a true Zs value and you will see a difference.
That with your R1/R2 reading and tripp times will confirm compliance.

 

[Guest77]

I would contact the manufacturer and ask. Personally I would be concerned, but the manufacturer may have a reason.

Chuckle, bet you would like to remove your comment now, you will soon enough I bet.

 

[TJ Anderson]

Hi all. I carried out an EICR yesterday. It was a BG split load consumer unit and when testing I noticed the zs values were coming out high.
I noticed on the 1 rcd there was 0.50 Ohms difference between feed and load side of rcd and on rcd 2 there was 0.60 Ohms difference. Just wondered if anybody else had found this and also does anybody think it needs further investigation. I know the zs doesn't really matter because the rcd Will still trip at 1667 ohms. However just wondered what everybody else thought because I've never come across it before. Thanks

I often note the impedance that has been added by RCD on my EIC / MW and always do if it takes the Zs beyond maximum satisfactory disconnection by MCB protected by it.

 

[ImpededLoop]

Chuckle, bet you would like to remove your comment now, you will soon enough I bet.

Chuckle, I'm not too big to admit that it looks like I'm wrong this time!

I'll add a line to the original to say it's rubbish....

 

[happysteve]

Some manufacturers' earth fault loop impedance testers ("MFTs") are more susceptible to this than others. Some call it "RCD uplift". It can be up to an ohm in some cases.

 

[telectrix]

to go to the extreme, i had BG comdemn an installation as he read out of limit Zs. my 1553 confirmed a Zs at sockets of 351 ohms. testing across L busbar and E in the CU gave 349 ohms. Ze on the upfront RCD incoming L and E was 0.2 ohms.