New RCD tripping

[kerching]

Having read the last few posts I ( and possibly others ) am intrigued as to what course/exams you did. Please don't take this the wrong way BUT there appears to be a lack of basic teaching of fundamentals

 

[Murdoch]

Please don't take this the wrong way BUT there appears to be a lack of basic teaching

edited that for you

its made worse by the people at the top assuming knowledge and not communicating downwards ......

 

[SPECIAL LOCATION]

its all about the  ££££££££££££££££££££££££££££££££'s

and Bums-On-Seats...

Nothing to do with passing on knowledge to make anyone competent!

:C

 

[Mell]

Hi kerching the courses I did was the short course organised by my company a month long course back in 2008, I had a basic knowledge of electrics but this course taught you practical installing, theory from ohms law to testing and inspection and building regs and more that I can't remember and at the end about 19 exams in 2 days don't ask me what they were all on but it was 17th edition and I found the course just skimmed everything taught you how to use the wiring regs book to source information but didn't teach us how to fault find just what results you should get and fill in the forms and recently did my 18th edition course and exam and also test and inspecting exam again the exam is to make sure you test safely and incorporate 3 faults that you must find all of which is city and guilds certification.

 

[OnOff]

You might get yourself a copy of Guidance Note 3 Inspection & Testing. Colloquially referred to as "GN3". Covers all your dead tests with big, pretty pictures. 

As aforementioned, anything by Brian Scadden is great (imho). What you need to know with no waffle. He was a nice man too. 

 

[SPECIAL LOCATION]

Hi kerching the courses I did was the short course organised by my company a month long course back in 2008, I had a basic knowledge of electrics but this course taught you practical installing, theory from ohms law to testing and inspection and building regs and more that I can't remember and at the end about 19 exams in 2 days don't ask me what they were all on but it was 17th edition and I found the course just skimmed everything taught you how to use the wiring regs book to source information but didn't teach us how to fault find just what results you should get and fill in the forms and recently did my 18th edition course and exam and also test and inspecting exam again the exam is to make sure you test safely and incorporate 3 faults that you must find all of which is city and guilds certification.

Ok Mell...  Not sure if you are aware or not.. But..

Back in 2005 when Part-P building regs was introduced to "tighten-up" on electrical safety in dwellings, there was a requirement for electricians to join competent persons schemes to be able to sign-off and notify their own work without the need for getting the local council building control involved..

To become a member various criteria were set including what level of qualifications were deemed necessary..

Any new starter doing a bog-standard recognised full time college course as part of formal employment training,

would typically meet these qualifications criteria upon completion of their full time course..

But there was a problem for lots of existing time-served-electricians already working in the trade..

who overnight technically became not competent because their qualifications were not fully up-to date..

Hence a whole variety of short courses were introduced to allow these electricians trained to earlier versions of reg's to get up-to-date..

BUT..  then for reasons I can only assume relate to greed and the desire for easy money..

Many training providers opened these courses up to completely unskilled, unexperienced, new starters as if they were a fully comprehensive package!!

Whereas in reality they were mostly regs-update-courses!

From what you describe I have a gut feeling this may be the type of course you did?

As if it was in 2008 it would tie in nicely with the 2005 onward purge for short-course candidates, money via bums-on-seats!!

Do you have any City & Guilds course numbers for the exams you took?

e.g.   C&G 2391

You can go to the city & guilds web-site to check what definition they give for the courses / qualifications you have..

https://www.cityandguilds.com

Guinness  

 

[Mell]

Hi special location yes I understand what your saying and yes my certification has got course numbers on them, will check in due course as the company holds my certificates.

Thanks

Mel 

 

[Mell]

Hi on off

I have guidance note 3 inspection and testing and know how to do my step by step testing as explained in previous threads and also know what results I require.

Thanks 

Mel 

 

[OnOff]

Hi on off

I have guidance note 3 inspection and testing and know how to do my step by step testing as explained in previous threads and also know what results I require.

Thanks 

Mel 

No offence mate but it clearly says in there about testing RCDs off load. 

 

[Mell]

To be honest on off I haven't read that bit in the book as I've never had a problem testing an RCD but the way I was taught was to use my metrel at the socket or accessory and do the RCD test and it would do the 6 tests but recently after being advised I did read up on RCDs and stood corrected it did say how to test an RCD off load, just a quick one, if you test an RCD off load and trips at 1 x load and trips at 5 x load as it should but then on half load it trips on the second cycle is that a defective RCD because it shouldn't and do the same test on the same board but on the other RCD and get a pass on the metrel. I think its a faulty RCD but I'm willing to be wrong.

Thanks

Mel 

 

[SPECIAL LOCATION]

Hi special location yes I understand what your saying and yes my certification has got course numbers on them, will check in due course as the company holds my certificates.

Thanks

Mel 

Why would your company hold your certificates?

In the normal run of things any exams/courses you take, the qualification should be in your name and belong to you..

I have copies of every certificate I have ever attained..  (including my cycling proficiency test that a took whilst at school !).

Unless your company had some in-house bespoke course tailored to meet their specific area of work...?

A previous employer I worked for had a "trimmed down" Novell Network installation course organised by a pucka training provider..

as we needed to set-up various desktop PC's on some LAN's for a few large companies..

But they didn't want us all being qualified and clearing off to become computer geeks...

Just sufficient knowledge to get the job done! 

 

[Mell]

I don't keep my certificates and they are in my name but I now they are safe and if customers want to see them then my company sends copies I don't need to keep them,  my company is an a v company not electrical we only occasionally install electrics when needed.

 

[SPECIAL LOCATION]

To be honest on off I haven't read that bit in the book as I've never had a problem testing an RCD but the way I was taught was to use my metrel at the socket or accessory and do the RCD test and it would do the 6 tests but recently after being advised I did read up on RCDs and stood corrected it did say how to test an RCD off load, just a quick one, if you test an RCD off load and trips at 1 x load and trips at 5 x load as it should but then on half load it trips on the second cycle is that a defective RCD because it shouldn't and do the same test on the same board but on the other RCD and get a pass on the metrel. I think its a faulty RCD but I'm willing to be wrong.

Thanks

Mel 

Re the off-load test requirement..

You will know, or should know some electrical equipment can have some natural leakage..

So if you were testing an RCD connected to circuit with loads with say 6ma of natural leakage already on the circuits..

how to you think that may affect your results?

You need to test & verify that the RCD itself is operating correctly..

which although 90%+ of the time may well be quite accurate using a socket.. 

IF.. there are any strange or unexpected results.. then you need to get back to just the RCD so you don't start chasing the wrong problem..

An RCD that trips at 15ma with no loads connected is oversensitive..

(Providing your meter is suitably calibrated and known to be accurate).

 

[OnOff]

To be honest on off I haven't read that bit in the book as I've never had a problem testing an RCD but the way I was taught was to use my metrel at the socket or accessory and do the RCD test and it would do the 6 tests but recently after being advised I did read up on RCDs and stood corrected it did say how to test an RCD off load, just a quick one, if you test an RCD off load and trips at 1 x load and trips at 5 x load as it should but then on half load it trips on the second cycle is that a defective RCD because it shouldn't and do the same test on the same board but on the other RCD and get a pass on the metrel. I think its a faulty RCD but I'm willing to be wrong.

Thanks

Mel 

A quick ready reckoner is swap the RCDs over assuming they're the same spec!

 

[Mell]

Hi special location

I did test the RCD without any loads and tripped at half the load, as for your question I wouldn't expect the RCD to trip at 6ma leakage as the RCD is a 30ma and won't trip till it senses over 30ma difference between line and neutral.

And on off yes I thought about swapping or trying another RCD but thinking about it I know the other RCD is working correctly so swapping is a better option.

 

[phil d]

Hi special location

I did test the RCD without any loads and tripped at half the load, as for your question I wouldn't expect the RCD to trip at 6ma leakage as the RCD is a 30ma and won't trip till it senses over 30ma difference between line and neutral.

And on off yes I thought about swapping or trying another RCD but thinking about it I know the other RCD is working correctly so swapping is a better option.

Just watch your thinking on tripping currents for RCD'S/RCBO'S, most of the ones I've tested trip at anything between 21 and 26 milliamps, so you need to think of that, incidentally which part of Shropshire are you in, anywhere near market drayton?

 

[Mell]

Hi Phil d

OK I'll keep that in mind I'm from Telford mate not far from you.

 

[SPECIAL LOCATION]

Re the off-load test requirement..

You will know, or should know some electrical equipment can have some natural leakage..

So if you were testing an RCD connected to circuit with loads with say 6ma of natural leakage already on the circuits..

how to you think that may affect your results?

Hi special location

as for your question I wouldn't expect the RCD to trip at 6ma leakage as the RCD is a 30ma and won't trip till it senses over 30ma difference between line and neutral.

I think you may have missed the point of what I was trying to illustrate to you..

You mentioned doing ramp tests.. (which although not required by BS7671 are a very good diagnostic test when doing fault finding)..

and doing the 15ma (1/2 trip current test),  which is required by BS7671...

If you were doing either of these with unknown loads connected that have a  bit of natural leakage..

this leakage can affect your test results..

e.g.

If you've got one of those basic polarity tester plugs that light up three neon's if it thinks the wiring is good..

due to the way they work there will always be some CPC current flowing..

( If you've also got a milliamp clamp meter, do a test and see what you get..   

I have a couple, one that is just neon's another that warbles as well..

One has a 4.2ma CPC current the other 3.1ma current.. )

So if doing a ramp test on a circuit with 5ma of leakage already present..

Your meter starts increasing its test-tripping current..

but the reading you get will be approx:  The RCD's actual tripping current, minus the existing leakage current.

And if you were doing a 15ma no-trip test...

Your test meter puts out a 15ma CPC current,

But the RCD sees both your meters test current AND any existing leakage current..

which if I had my two test plugs connected at the time would be approx 15ma+4.2ma+3.1ma= 22.3ma

Which technically is within the valid range for a 30ma RCD to trip..   

(Remember RCDs must trip fast enough with a 30ma or greater CPC current.

But they can also trip at lower values providing it is above 15ma..

see reg 531.3.2. & section 11 of On Site Guide)

I have seen many real world installed RCD's who trip around 24ma upwards..

SO its not rocket science to see that a connected load with 8ma of natural leakage could mess up your readings if testing with loads still connected!

( They can also have a negligible effect upon the trip times..

But in the real world it is very unlikely that the RCD is anywhere near its max permitted trip time, so that isn't a problem. )

Guinness   

 

[phil d]

@SL every rcd/rcbo I install now gets a ramp test, first with no outgoing cables connected and again with the outgoing cables connected and at a socket rather than the board, there are a couple of reasons why I do this.

1) I have on a couple of occasions had new devices that have tripped correctly with nothing connected but not tripped when 'in circuit' , and I always like to know that it trips at a socket outlet, after all if it trips at the board but not at a socket it isn't much good is it?

2) The ramp test gives me a baseline of the actual tripping current of the device (not in circuit) and also a baseline of the leakage in any circuit it is connected to, if for example I know that I have say 8ma of leakage on a circuit today and get called back in 6 months to a fault, and find I now have 15ma of leakage on that circuit I know something is amiss and it gives me a starting point, as I'm sure you well know.

As we' ve said it's only with experience that you learn all the little tricks to fault finding, being a spark and wiring stuff isn't that difficult, it's the fault finding that seperates the men from the boys isn't it. It's like measuring leakage current, so many people don't know how to do it, they clamp the circuit earth rather than the live and neutral, it's as though they forget that if a circuit has metal back boxes, which most have, some of the leakage will go through the box, not come back down the earth.

Hopefully the OP will be learning something from this discussion that will help him in future.

 

[SPECIAL LOCATION]

@SL every rcd/rcbo I install now gets a ramp test, first with no outgoing cables connected and again with the outgoing cables connected and at a socket rather than the board, there are a couple of reasons why I do this.

1) I have on a couple of occasions had new devices that have tripped correctly with nothing connected but not tripped when 'in circuit' , and I always like to know that it trips at a socket outlet, after all if it trips at the board but not at a socket it isn't much good is it?

But...  You meter is only outputting a very small fault current to test the RCD itself..  

Which may not be sufficient to operate the RCD when tested out on a circuit due to other circuit losses through fixed equipment that may not have their neutrals isolated..

In your scenario where trips ok with meter at RCD..  But doesn't trip at a socket out with meter..

( which i have seen myself numerous times )..

Have you actually gone on to test it with a more realistic fault..

e.g. back in earlier GN3 days, before RCD's became more common place,

a note used to suggest using a 15watt pygmy lamp wired between live & earth to verify an RCD tripped..

(I still have plug & lamp & pretty much always trips the RCD at a socket even if my meter doesn't) 

15Watt lamp is a tad higher than the basic 30ma fault.  (nearer a 65ma fault)

AND very very very rarely would a real fault be only 30ma.

and/or have you tried a Zs through the RCD to see if it trips?

You can end up chasing yourself up your own rear-end trying to get your meter to give a valid RCD test at all points on a circuit..  

Which is probably why the regs DONT ask you test in circuit! 

:coffee