When testing a circuit protected by RCBO, do you test at the DB or furtherst point

[Pete999]

you'd think so, but IR testing with modern digital testers can be hit and miss. had several cases of trapped L conductors showing >299Meg. on test but tripping MCB when energised. also had a couple of N-E fault stopping RCDs working . tested fine, >200Meg. .as a last resort on one of the latter, i used old wind up megger. that showed 15K ohms. think it's the fact that the old wind up jobbies can push out higher current. . moral here is don't mock the oldies. (you listening pete? old school beats the new tech.... hands down).

Agree

 

[GBDamo]

I just press the test button, am i doing something wrong then?

 

[l4urence]

I always test at the furthest point with RCBO's, just do.

 

[Toneyz]

I always test at the furthest point with RCBO's, just do.

I just test at the point that is easiest for me.

 

[l4urence]

I just press the test button, am i doing something wrong then?

I think when AFDD's, RCD's and MCB's are combined in one unit - that's all we'll be able to do. AFRCBO's? and then throw in an SPD somewhere.

 

[Lucien Nunes]

I think the point that keeps getting overlooked is that (barring Tel's odd insulation fault thing) a typical circuit that passes the dead tests is not going to make any difference to whether the tester performs a valid test on the RCD, nor affect whether a functioning RCD will successfully provide additional protection. A long, highly capacitive circuit will add some leakage current and may affect the result, making the RCD look more sensitive than it is in itself, although without impairing the protection because it genuinely will trip at the measured lower current when in use on that circuit. But that only impacts whether it is worth disconnecting the circuit, not where to attach the tester leads while the circuit remains connected. Leaving loads connected is a different matter and can significantly affect the results.

An analogy would be to argue that since windscreen wipers are used to wipe rain off, the only valid test for the MOT would be to put the car outside and wait for it to rain, and see if the wipers removed it. Overlooking the fact that they remove water from any source with similar efficiency, just as an RCD responds to a particular injection test current with similar efficiency regardless of where on the circuit it is injected.

The thing about RCDs that don't respond to a test at the screw terminals, is that they use the circuit conductor, as it passes through the plastic housing into the terminal, as part of the balance transformer. It has nothing to do with the length of cable etc, only where the test probe current physically enters the device. It must go through the cable hole, not the screw hole.

 

[littlespark]

So a device that is;-

Arc fault detection
Residual current
Surge protection
Equipment

Sounds like a bit of a pain in the A.R.S.E

 

[l4urence]

So a device that is;-

Arc fault detection
Residual current
Surge protection
Equipment

Sounds like a bit of a pain in the A.R.S.E

I tried so hard to get that to work but I had a couple of beers... well done. Exactly

 

[Simon47]

i used old wind up megger.

I have an old "hold the button and what the needle does" IR & resistance tester that dad picked up when it was being ----ed out at work. In my rental flat, I can use that to test the whole installation for IR as it will provide voltage for long enough to charge caoacitance (eg in PIR lights that are inconvenient to disconnect) - but MFT simply doesn't apply voltage for long enough and gives random results.
IMO that's far preferable to all the disturbance that would be caused by disconnecting all the circuits from their RCBOs. At some point you have to consider if the testing is introducing more risk than it might be mitigating ? And given the terminal design (I'm thinking of those long RCBOs where the L&N are one above the other so one is deeply recessed and impossible to visually inspect) of some devices, disconnecting them is creating a risk that isn't warranted IMO.

 

[nowaypedro]

I'm with Spin and Tel

I'm with Spin, Tel and Pete

 

[stevethesparks]

Ridiculous change to the regs IMHO! A working 100mA RCD will pass as a 30mA one.

100% agree

 

[telectrix]

100% agree

sure that's not 30%?

 

[PEG]

......and i thought YOU,were the popular front of Judea...

 

[Deleted account]

......and i thought YOU,were the popular front of Judea...

No the people popular front...

 

[PEG]

No the people popular front...

Yeah....he's over there...

 

[Deleted account]

Splitter

 

[telectrix]

......and i thought YOU,were the popular front of Judea...

no, i'm with the Judean Mafia. we gave Brian an offer he couldn't refuse, but he got a bit cross.