A puzzling one from today

[M107]

CU change all rcbo board (sorry SBS Dave customer had a contactum unit) for a mate who has just purchased the property.

Cooker circuit 6mm² T/E on 32a 30mA rcbo no cooker connected (using gas hob/oven) ccu with socket no neon. 

R1+R2 = 0.12 ohms

IR = >200Mohm

Zs = 0.59 ohms

RCBO test x1 = 310mS

RCBO test x5 = 50mS

Changed RCBO same results

Tested at RCBO load terminals same results

Removed circuit conductors tested at RCBO terminals 

x1 = 28mS

x5 = 28mS

Any ideas........not something I've ever had happen before, I'm thinking there maybe another load on the circuit some where, courgetteed if I could find it though.

 

[ProDave]

That would make Ze a bit on the high side wouldn't it? What type of earthing?
 

 

[M107]

TNCs  measured at 0.27 ohm

 

[poni]

i had that the other day, it was my meter was set on DC , when I changed it to ac the RCD tripped in 0.032s and 0.011s

 

[kerching]

Cold meter that needed to,warm up a bit?

sounds daft but I have had that a few times

and many many years ago a Central Heating prog that threw every test a total wobbler

 

[ProDave]

TNCs  measured at 0.27 ohm

where's the other 0.2 ohms then?

Ze 0.27

R1+R2 0.12

Zs 0.59

 

[Revved Up Sparky]

Has your tester been calibrated ? 

Best Regards

RUS

 

[steptoe]

Ye, my meter is a bit funny on these really cold mornings until it warms up a bit.

On an aside tho, you are supposed to test an RCD/RCBO with ALL outgoing circuits disconnected anyway, 

 

[Sidewinder]

As Steptoe says, plus you've got some capacitance there from somewhere.

How long is the cable run to the cooker?

 

[Evans Electric]

Ye, my meter is a bit funny on these really cold mornings until it warms up a bit.

On an aside tho, you are supposed to test an RCD/RCBO with ALL outgoing circuits disconnected anyway, 

You made me think there Stepps  so I just had  a look in my NICEIC  Inspection & Testing  book .  

It says  "  Connect the instrument to the load side of the RCD  with the load disconnected .  The connection can be made at any convenient point on the circuit such as a socket outlet ". 

Personally I've often done RCD tests at the same place as  Zs etc .

 

[Sidewinder]

I do as well Deke, if it passes at the extreme point of the circuit with all the associated capacitance etc. then it will be fine at the device, IMHO.

However, the test, is to test the device, and the device only.

It is not an in-circuit test.

 

[steptoe]

In all honesty I just test wherever convenient,

I did ask Niceic bod about this when I was a QS, and put it direct to him was it because of all the dodgy PV installs preventing the RCDs tripping out, he couldn't answer me.

 

[Murdoch]

Test at the RCBO only? And / or remove the cooker socket / switch and test the cables..... older CCU's can have hopeless connections .....

 

[M107]

Circuit length is in the region of 15 - 20 meters.

Yes meter is cal'd

Was a cold morning so could be an issue.......should have tried new batteries as well I suppose.

As I stated I did in fact test the rcbo with out the circuit connected & it is working fine. I hadn't come across the issue before hence asking.........thanks for the input everybody.  

 

[M107]

Test at the RCBO only? And / or remove the cooker socket / switch and test the cables..... older CCU's can have hopeless connections .....

Did all that in the exact same order in fact.........

Just the rcbo = pass test

Connected circuit but no ccu = fails test

 

[Murdoch]

If it works without the cable attached, and doesn't with the cable, that would point at a fault imho

 

[Rob_the_rich]

The times for the x1 and x5 tests at the rcbo terminals with nothing connected are the same. Shouldn't there be a couple of short wires connected to the rcbo, and the tests done from their ends? I seem to remember that if a test is done from just the terminals, it could affect the results, for some reason, possibly because it is within the confines of the device.

 

[Sidewinder]

Some devices have their sensing coils in the terminal tunnel, so if you put your probe on the screw head, you don't pass current through the sensing coil.

So, it may be necessary to connect short flyleads, however, the main circuit is required to be disconnected from the device by the testing procedures given in BS7671 & associated guidance.

 

[Rob_the_rich]

That makes sense, thanks SW. I suppose the design of rcbos, with one terminal above the other, and space restrictions within the device, makes them more likely to need fly leads.

So it could be that the rcbo (and the replacement one) is faulty.

 

[Rob_the_rich]

Thinking more on this, is it because the screw is not electrically continuous with the backplate (to which the sensing coil is attached)? would this be true with cage type terminals?

Edit; oops, then it would fail the test, doh!