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Topic Title: RCD's
Topic Summary: Tripping
Created On: 02 January 2013 07:24 PM
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 02 January 2013 07:24 PM
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ady1

Posts: 767
Joined: 19 April 2005

I'm wiring a shed and a garage - both with separate supplies and boards, to the house.
As a temporary (till the path is dug up) im using a plugtop on flex to an existing socket in the house.
I dont understand why, when i press the test button on a 61008 RCCB
it doesnt trip the house board, but when (on the other board) i press the test on a 61009 RCBO it does trip the house board.
I presume its because the RCBO is single pole....
Am i right ?

Regards
Ady

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Resistance is futile.
 02 January 2013 09:42 PM
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ebee

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With two RCDs in cascade a fault current (or test current) could trip either one or both RCDs depending upon the actual operating times.

That is why cascading we would separate by sensitivity and also by time.

Say RCD1 = House, RCD 2 = Garage & RCD 3 = Shed
then RCD1 & RCD 2 cascade for Garage , RCD 1 & RCD 3 cascade for Shed.
In each case the fastest of the 2 RCDs in cascade will trip and then the slowest RCD of the two might not trip (It might still trip if it has started its tripping function before the other RCD removes the fault though)

This seems to be what you have experienced.

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Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
 02 January 2013 10:16 PM
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leckie

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If you are saying that you have taken the supply from an rcd protected main db, why do you want additional rcd/rcbo's at the sub db's?

Edit
Bit of a misread
I assume your plugging into 30mA protected socket, see previous post. The rcd's are not discriminating
 02 January 2013 11:03 PM
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ady1

Posts: 767
Joined: 19 April 2005

Thats how it is Ebee...
As i say its only a temporary situation, but if i knock the test button whilst working in there i have to get the tennant to let me in the house. I realise i could have done away with the rcd's in the garages as they are on plugs for now (in rcd'd sockets).
Guess i will have to be more careful....
Nothing to do with the double pole RCCB then
Shame we cant fit a diode on the test button side...... ?????

Thanks guys

Ady

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 02 January 2013 11:14 PM
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Legh

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I've had a similar problem when using a B&Q plug in RCD to BS7071.? It appeared not to be compatible with BS EN 61008

The one I used had no discrimination under normal working conditions that's even before any 'normal' fault current.

Legh

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 03 January 2013 07:05 AM
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ebee

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If you mean the difference in RCD trip times between our good old BS types and the newer BS (EN) types then that is not really the issue (Although I must admit the time delay parameters leave me a bit dismayed).

Any cascading devices must be less sensitive the further up the supply you go otherwise it will be possible that either or both will trip under fault (or test - that is test button or metered test).

They must be separated in two parameters
1/ sensitivity (recommended to be by at least a factor of three 10 &30, 30 & 100, 100 & 300)
and also
2/ by time - standard time then delayed time.

That is the only way to reasonably expect discrimination to occur.

If discrimination is a issue then it must be addressed, if not then fair enough.

On non time delayed RCDs the difference between trip times at rated current was within 200ms for BS and within 300ms for BS (EN). Half trip and five times trip are the same.

Although in practice I suspect they all conform to both standards trip times under test conditions as I`ve yet to see one take over 100ms to trip (or anyway near it) at I delta N on an unloaded circuit

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Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik

Edited: 03 January 2013 at 07:16 AM by ebee
 03 January 2013 09:19 AM
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AJJewsbury

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All the "T" buttons I've seen on RCCBs/RCBOs short (via a resistor) L on one side of the device to N on the other side - i.e. they create an imbalance without involving the c.p.c. or real leakage current. In this case other supply-side RCDs shouldn't see any imbalance and so shouldn't trip.

If your RCBO T button is causing upstream devices to trip then either it's T button is wired to the c.p.c. (which I guess is possible, although I've never seen it before), or there's a wiring fault (perhaps N-PE) which is allowing the test current to exit the circuit somehow.

Testing with an RCD tester does of course create a true leakage current, so the discrimination issues would be seen then.

- Andy.
 03 January 2013 03:46 PM
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ady1

Posts: 767
Joined: 19 April 2005

Thanks Andy
You may be onto something....
The RCBO is a mem 2000 with the rcd kit on it - could that be why ?
I'm wondering - wouldn't a N-PE fault show on my polarity, EL and PFC tests and RCD for that matter ?
Regards
Ady

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 03 January 2013 03:46 PM
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ebee

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"All the "T" buttons I've seen on RCCBs/RCBOs short (via a resistor) L on one side of the device to N on the other side - i.e. they create an imbalance without involving the c.p.c. or real leakage current. In this case other supply-side RCDs shouldn't see any imbalance and so shouldn't trip. "

Spot on Andy, I should have thought of that, the test button operation should not have that effect as its a Ph = N balance external to the coil under test.

Mmm what about those RCBOs that have a functional earth to ensure operation (how much do they draw? not much I`d have thought)

-------------------------
Regards,
Ebee (M I S P N)

Knotted cables cause Lumpy Lektrik
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