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Topic Title: RCBO trip time issue
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Created On: 03 October 2013 08:39 PM
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 03 October 2013 08:39 PM
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nlasen

Posts: 164
Joined: 23 December 2006

Evening all. Bit of a strange one here,

Councils testing company raised a D1 defect for an rcbo not tripping within permitted limits.

Board is a dorman smith loadlimiter.
Went to site and tested to prove issue , sure enough greater than 310ms and greater than 50ms

Changed unit for brand new one and tested to confirm , readings the same from the socket. failed again. Thought it might be the socket, so tested direct from the rcbo terminals , Failed aswell. Changed again for a known working one from another DB , and same results.

Thought maybe tester faulty so tested an adjacent RCBO and tested and passed with no issues.


Anyone got any clue WTF is going on and what the next steps would be apart from changing for another new unit . Have considered swapping it to another phase and see what happens.
Just to add all other tests pass, Zs 0.41 ohms

Nick
 03 October 2013 09:01 PM
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alanblaby

Posts: 352
Joined: 09 March 2012

I've had this a number of times on domestic installations.
Nearly always traced to a hidden appliance, or possibly a PIR sensor.
When the neutral cable for that circuit is removed (safely), and tested again at the RCBO, there is usually a good result.
 03 October 2013 09:02 PM
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leckie

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Blimey Nick, now that is confusing! As I was reading the post I thought Zs must be open circuit, but no, you have tested. The test for Zs includes the line and earth path so if the reading is ok that should confirm the line and earth connections are ok.

Need someone clever to reply to this, which excludes me! But I will have a think. It's not going to be two faulty units, or if it is, that's odds that win the pools ( or lottery for the younger ones).
 03 October 2013 09:04 PM
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leckie

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Check line to neutral IR and short circuit fault current. Can't work out If this is relevant, but it's something else checked out!
 03 October 2013 09:14 PM
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JonSteward

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Did you test with the load disconnected from the RCBO?
 03 October 2013 09:18 PM
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dg66

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Try disconnecting the circuit and test at the RCD termials,afterall all you should be tseting is the RCD. Also ensure the supply side RCD connections are not loose.

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Dave(not Cockburn)
 03 October 2013 10:02 PM
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nlasen

Posts: 164
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did test at the RCBO but with the cct connected, if i test it with the cct disconnected and it passes what are your thoughts to the cause.

I was always told to test RCD and RCBOS at the field end as that would be the nearest simulation to real life conditions, as opposed to testing at the rcbo with loads disconnected.

Nick
 03 October 2013 10:14 PM
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dg66

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Read GN3 and that confirms that you should test the RCD at its terminals.As previously mentioned connected loads could affect readings

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Dave(not Cockburn)
 03 October 2013 11:17 PM
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alancapon

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Originally posted by: nlasen
did test at the RCBO but with the cct connected, if i test it with the cct disconnected and it passes what are your thoughts to the cause. . .

What does your RCD tester measure to determine whether the device has tripped?

What may affect its reading?

. . . I was always told to test RCD and RCBOS at the field end as that would be the nearest simulation to real life conditions, as opposed to testing at the rcbo with loads disconnected. . .

What does the guidance note say on the subject?

Regards,

Alan.
 04 October 2013 07:33 AM
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Rulland

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I hate riddles.

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Those who make no mistakes do very little work!!......
 04 October 2013 07:36 AM
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ebee

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"I was always told to test RCD and RCBOS at the field end as that would be the nearest simulation to real life conditions, as opposed to testing at the rcbo with loads disconnected. "

Me too - Preferably radials at far end and rings near to midpoint.

However, guidance now clearly tells us to check the RCD itself and not influenced by the load.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 October 2013 08:26 AM
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Legh

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It could always be a dodgy batch of RCBOs.

Once all variations have been tried, you might consider contacting the wholesaler and then the manufacturer's technical dept.

Legh

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 04 October 2013 08:36 AM
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ebee

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"It could always be a dodgy batch of RCBOs. "

It could but testing the RCD in isolation would confirm this

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 October 2013 09:09 AM
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weirdbeard

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Originally posted by: Rulland

I hate riddles.


Hi rulland - heres my take on what alan said:

When you do an RCD test, pressing the button starts a timer which tells you how long it takes for the voltage to drop to zero across the outgoing terminals of the device.

Electrical equipment and cables all have a certain amount of capacitance which means they get charged up and when the supply is switched off, the capacitance will feed back to the outgoing terminals and thus the rcd tester, causing the testers timer to run on a bit longer than it actually took for the device to operate.

So, for the OP, i would suggest as others have done that the device be tested with the cables disconnected and see what happens.
 04 October 2013 09:54 AM
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perspicacious

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"Thought it might be the socket, so tested direct from the rcbo terminals , Failed aswell."

"did test at the RCBO but with the cct connected, if i test it with the cct disconnected and it passes what are your thoughts to the cause."

The issue appears to be not with the RCBO, but with conflicting OP stories in his test procedure......

Regards

BOD
 04 October 2013 11:16 AM
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ebee

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I`m not sure that I see a conflict there BOD.

I think he`s checked each end but with the cct connected in each case.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 04 October 2013 01:49 PM
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Rulland

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Originally posted by: weirdbeard

Originally posted by: Rulland



I hate riddles.




Hi rulland - heres my take on what alan said:



When you do an RCD test, pressing the button starts a timer which tells you how long it takes for the voltage to drop to zero across the outgoing terminals of the device.



Electrical equipment and cables all have a certain amount of capacitance which means they get charged up and when the supply is switched off, the capacitance will feed back to the outgoing terminals and thus the rcd tester, causing the testers timer to run on a bit longer than it actually took for the device to operate.



So, for the OP, i would suggest as others have done that the device be tested with the cables disconnected and see what happens.


Cheers Weirdbeard, that clarifies without searching for an answer,
Richard.

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Those who make no mistakes do very little work!!......
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