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Topic Title: RCD test on RCBO
Topic Summary: Never done it before!
Created On: 07 February 2008 06:33 PM
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 07 February 2008 06:33 PM
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beemer

Posts: 188
Joined: 23 February 2007

Hi...I have never done a RCD test on a RCBO before and wanted some confirmation:
I will be replacing a shower unit, like for like, however, I will be replacing the MCB for a RCBO.
I am assuming that in order to test RCD times on my shower, I would use my test probes on the live terminals inside the shower unit, and reset RCBO when required at 50% of the rated tripping current and a further test at 100% of rated trip current.
Have always done sockets before so this is a first for me.
Constructive advice much appreciated...thanks

Dave
 07 February 2008 07:19 PM
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charlieroad

Posts: 150
Joined: 24 September 2007

Hi Beemer,
What tester do you have? don't forget when you are testing you need to test at 0 deg. and 180 deg.
50%, 100%, 500%. Record the slowest trip in each test. Bear in mind the 50%, should reliably hold - not trip.
I always test at the device, not switched on. This is made much easier with the Megger 1552 as it has an auto test, so you set it up at the device and just keep reseting the RCBO until it stops tripping. The results are recalled on the unit; it takes about 3 minutes from start to finish.

Make any sense or am I waffling?
 07 February 2008 07:40 PM
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beemer

Posts: 188
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Originally posted by: charlieroad

Hi Beemer,

What tester do you have? don't forget when you are testing you need to test at 0 deg. and 180 deg.


Hi Charlie,

I have a Meterel Easitester, so test is done automatically.
testing at the RCBO will be much easier for me.....thanks for the advice.

Dave
 07 February 2008 07:41 PM
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Testit

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Joined: 06 August 2007

clearly I am confused.. u tested RCDs?

0 and 180deg, 1/2, 1, 5 times

Shouldnt matter where u test it but I would do it at the end of the radial on auto for my meter and just reset the rcbo. otherwise its the same as an rcd just with an mcb, except usually the switching off to reset is slightly more energetic..

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 07 February 2008 07:58 PM
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charlieroad

Posts: 150
Joined: 24 September 2007

Hi all,
Yes I did mean without the current using equipment turned on! Obviously the RCBO would need to be switched on
0deg. and 180deg. tests are carried out automatically but with the older testers you need to select th settings manually.
 07 February 2008 11:41 PM
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dg66

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One thing everyone seems to have overlooked,when all time tests are complete press the test button to ensure this functions correctly

-------------------------
Regards

Dave(not Cockburn)
 08 February 2008 07:19 AM
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Jaymack

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Originally posted by: dg66

One thing everyone seems to have overlooked,when all time tests are complete press the test button to ensure this functions correctly


I think it's preferable to press the button before you start the electrical tests. Any "stiction" of the mechanical assembly could affect the readings i.e. taking longer to trip.
 08 February 2008 07:33 AM
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naughtymoose

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Jaymack
You could use that argument the other way around.

I always test the RCD with the tester. If it doesn't function properly because it is 'sticky' or 'tired' then it is a good indicator that it has never been 'exercised' by the installation user.

Doing it this way provides you with good information to teach the installation user about self-testing their RCD.

Of course, they never listen...
 08 February 2008 07:44 AM
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GeoffBlackwell

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I think it's preferable to press the button before you start the electrical tests. Any "stiction" of the mechanical assembly could affect the readings i.e. taking longer to trip.


That is exactly why you should not do that.

Stiction of the RCD, as you put may kill you If the device is prone to slow first time operation for whatever reason you need to find out.

I have made numerous posts on this - apparently to no effect. I also made recommendations about test procedure to the Electrical Safety Council before they completed their recent study on RCDs - again to no effect.

If you wish to find 'stiction' (and remember if your hanging on the end of this circuit when it happens you will wish that you had) you should test in the following sequence.

DO NOT OPERATE THE RCD BEFORE TESTING

5 * I delta n
1 * I delta n
1/2 * I delta n
Test Button

Any other sequence risks masking a major concern with RCDs which needs to be addressed if they are to provide the level of protection we all would like from them.

Regards

Geoff Blackwell
 08 February 2008 08:14 AM
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normcall

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Strangely, I always test the other way (apart from the test button which is always last).

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Norman
 08 February 2008 08:36 AM
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GeoffBlackwell

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Originally posted by: normcall

Strangely, I always test the other way (apart from the test button which is always last).


Well it is strange because the 5 * I delta n test is the most onerous as far as 'stiction' is concerned. It is this test that requires operation within 40 ms.

Do it first and you will uncover any tendency to operate more slowly on the first test. Do it second and you will not - because you have 'freed' the mechanism.

Regards

Geoff Blackwell
 08 February 2008 08:37 AM
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khales

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I was taught to do the 30mA test first, and if the device didn't trip to take that as evidence that the user probably wasn't exercising the device at the recommended periods. In which case use the test button to free it up, then do the full range of tests and finish off with a tactful 'telling off' for the customer.

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khales
 08 February 2008 08:39 AM
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khales

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Originally posted by: GeoffBlackwell...the 5 * I delta n test is the most onerous as far as 'stiction' is concerned....


I'm unconvinced. Surely the 5 * I delta n test produces a greater out-of-balance force on the mechanism and is therefore more likely to overcome any stiction?

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khales
 08 February 2008 08:39 AM
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GeoffBlackwell

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I was taught to do the 30mA test first


Why ?

Regards

Geoff Blackwell
 08 February 2008 08:46 AM
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Jaymack

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Originally posted by: GeoffBlackwell

That is exactly why you should not do that.


I would surmise that the lubricant used on the latching mechanism, would emulsify over a period of time, if the RCD was tested by operation of the pushbutton at frequent intervals - this wouldn't happen; and the RCD would have a higher probability of operating correctly every time. I think that very few people operate the RCD, at the stated 3 month intervals even though there is a notice at the DB - because of apathy and the inconvenience.
I have made numerous posts on this - apparently to no effect. I also made recommendations about test procedure to the Electrical Safety Council before they completed their recent study on RCDs - again to no effect.


Probably because they don't agree with you! the NICEIC training DVD on Inspection and Testing, advocates an initial pushbutton test to "prelubricate" the mechanism

Jaymack
 08 February 2008 09:04 AM
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daveparry1

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>I'm unconvinced. Surely the 5 * I delta n test produces a greater out-of-balance force on the mechanism and is therefore more likely to overcome any stiction? <

That's the way I see it too, surely the 5x test results in a more 'energetic' trip thereby freeing-up the mechanism so we wouldn't know whether it would have tripped at 1x after a period of inactivity? Please correct me if this is wrong Geoff,
regards,
Dave.
 08 February 2008 09:05 AM
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GeoffBlackwell

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"They don't agree with you" - well maybe, but there is another story buried in there somewhere - however, on to things we can say without fear of legal action .

You appear to accept the possibility of this problem so why not advocate a test method that has some chance of uncovering it ?

If this problem remains masked it does no service to anyone. The solutions to it are problematic - but that does not mean that we should not make any effort.

Solutions
1) Better education of the user - unlikely to be very successful
2) More testing - yes please that means more money for me.
3) Better RCD design - not my area but I expect some improvements are possible.

Regards

Geoff Blackwell
 08 February 2008 09:11 AM
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GeoffBlackwell

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Originally posted by: khales
I'm unconvinced. Surely the 5 * I delta n test produces a greater out-of-balance force on the mechanism and is therefore more likely to overcome any stiction?


The 'stiction' - to use Jaymack's terminology - occurs in the mcb part of the mechanism and, IMO, is not affected by RCD test current.

RCDs are just mcbs with an extra sensor and actuator. MCBs probably don't suffer too much from this effect when operating in overcurrent mode because the energy flows are much higher and the device would be under thermal and magnetic stress - as such it would far more likely to trip.

Regards

Geoff Blackwell

Edited: 08 February 2008 at 09:13 AM by GeoffBlackwell
 08 February 2008 02:08 PM
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normcall

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"You appear to accept the possibility of this problem so why not advocate a test method that has some chance of uncovering it ?"

When you get to my age young Geoff Blackwell, you will realise that us old bits of kit can move fast when we have too - but not if only gently pushed.
Like old RCDs, 500mA would make me move and overcome any stiffness, but 30mA might not - and that's what we are testing at the end of the day.

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Norman
 08 February 2008 03:21 PM
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GeoffBlackwell

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Well old Norman - As I understand it (I have one in bits in front of me right now ) - RCDs sense the out of balance current and then:

1) in the electro-mechanical types - trigger a sensitive actuator which then operates the switching mechanism. (Note: it was the development of this actuator that kick the whole thing off.)

2) in the electronic types - trigger a differential system (amplifier or similar) that then drives a solenoid, that then trips the switch mechanism.

In both cases the actuating force is not directly related to the out of balance current (unlike an mcb in overcurrent mode).

So us young ones might just know a thing or two - or maybe not .

For anyone else reading this - Norman must be at least 1 or 2 years older than me - we were once both at the same place in the dim and distant past.


Regards

Geoff Blackwell
IET » Wiring and the regulations » RCD test on RCBO

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