x5 RCD test fail.

[EdwardCurrent]

Hello All.
I understand that an RCD/RCBO should be tested 'at source', but given the 'chance' I tested out a friends RCBO with the 'plug in' lead. The EFLI was .5, (triplock), half rated no trip, 1X 21 ms. 5X >40ms.
Would I be correct in thinking, that the contact voltage that arose during the 5X test exceeded the 50v allowance. And that insulation resistance tests should be carried out asap ?

I understand that this shows my lack of testing experience/knowledge but am trying to understand the basics before embarking on a daunting 2391 'max day ' course. It's all too easy for me to 'read' procedures but I know I need much more 'hands on' experience I need.

Respect as always.

Ed

 

[securespark]

There are many pieces of equipment in circuit that could affect the test gear.

Do the test again at the CU before you write off the RCD as faulty.

Disconnect the outgoing neutrals (or if you can disconnect the N feed from the RCD to the busbar, so much the easier. Then switch off the MCB's. Now you can test in isolation from the outgoing terminals of the RCD.

 

[EdwardCurrent]

Cheers Secure.

Forgot to mention a ramp test @22mA . MEM 32A/30mA RCBO.

Its an RCBO so I will disconnect all other neutrals ?. It is niggling me now as to what 'could' be the cause of this result ?

Would earth leakage be less likely given the 1X result ??

For my own help, am I on the wrong tracks with 'contact voltage' being a problem if everything was to be unplugged on the ring in question, and the 1X passed and the 5X failure ?

Apologies and thanks for the help.

Ed.

 

[EdwardCurrent]

Hello All,

I see a lot of clever sparks here atm, and wondered if someone could advise me on my 'contact voltage' theory ?

Sorry to be a pain, but if I don't ask I wont know .

Respect and thanks.

Ed

 

[oharaf]

I have had a similar problem with RCBO's and using the socket lead when testing.

The advice of testing isolation is a good one, do it at the cu before getting shut.

This has solved it for me on 2 occasions and identified a dodgy RCBO on another

 

[Spark123]

Would I be correct in thinking, that the contact voltage that arose during the 5X test exceeded the 50v allowance. And that insulation resistance tests should be carried out asap ?

Don't think it will be anything to do with IR.
An RCD tester operates by sending mA down the earth path - if there is anything connected to the system it can corrupt the testers reading if it takes time to die off.
So need to test it in isolation.
Also before doing RCD tests you must always ensure the EFLI is sufficient as if it isn't under test conditions you may end up with mains voltage apparrent on exposed conductive parts (that question pops up on the 2391 exam from time to time too )
If the product of Zs and fault current exceeds 50v a modern tester may abort the test.

 

[EdwardCurrent]

Thanks very much for your help.

I was using a Metrel easitest and although the RCBO tripped, it gave a 'fault beep' and displayed >40ms.

So, when these 1X, 5X etc tests are carried out at the rcd/rcbo, with it in isolation, the results are not the same as will be found at any point of the circuit they control, and depending on if any appliances etc are connected to that circuit, then disconnection times taken around the circuit will vary.

Wouldn't it make more sense to find the 'working' circuits disconnection time, as this will be more relevant in the event of a fault during normal use ? This RCBO may disconnect correctly in isolation, but with loads connected 'may' not, and thats ok ???

Once again sorry. But I am easily confused.


Ed

 

[Spark123]

Not really as the RCBO is protecting the fixed wiring - you can plug almost anything in and to try to take that into account will be a nightmare!
You can also get mis readings from items being connected which naturally leak to earth such as washing machines which have filters on the mains.

 

[dunny123]

I have had some strange results testing RCBO's, where when testing directly at the rcbo terminals with probes at the CU can't get a trip at x1 or x5, but works fine when testing at the end of the circuit.

Anecdotaly, I was informed by an NICEIC inspector that you should never shorten the leads as supplied on an RCBO as they are balanced and with a set impedance which if shortened will affect the operability of the device. Anyone heard this/can corroborate this ?

 

[securespark]

Yup, [edit] Spark 123's right. If you talk to any manufacturer, they recommend testing in isolation. TBH, I sometimes test from a socket outlet and if it passes, I'm happy. But if not, I always test in isolation before writing off the unit.

I have a unit very similar to yours. It is well known to fail RCD's or RCBO's when appliances are in circuit.

So, with your RCBO, disconnect the outgoing connections and test in isolation. Only then will you know the true readings.

As an aside, on your kit have you a function that allows you to test individually, rather than the "autotest"?

IE, can you test the 5x on its own? Sometimes I find this helps. Try it and see what happens.

I can see exactly where you're coming from about the "working circuit's disconn time". I had a Birmingham spark ask me exactly the same during a training brief I did.

As I said to him, the correct method is to test in isolation & as long as the circuit complies in terms of EFLI limitations, the device will operate correctly.

 

[BS3036]

If you have any equipment connected to the circuit under test, particularly any with a switched mode power supply, there will be a 'Y' capacitor connected between line and earth, designed to reduce EMC problems.

When your tester tries to draw current through the RCD, part of that current will instead be drawn from the Y capacitor, and therefore not pass through the RCD.

This will potentially increase the operating time.

 

[Pensdown]

Anecdotaly, I was informed by an NICEIC inspector that you should never shorten the leads as supplied on an RCBO as they are balanced and with a set impedance which if shortened will affect the operability of the device. Anyone heard this/can corroborate this ?

Did you question the NIC chap?

Because there are so many up stream variables, technically, I can't see how it would make any difference. But if you're worried give the manufacturer a ring.

 

[securespark]

Yeah, I chewed this over with my Tech Manager today. He said much the same. That there are so many variables on that side of the installation that shortening the leads should not make any difference.

But I suppose it's a question best put to the mfrs, as Pens says.