RCD - Why does this Happen?
 

Hi all

Quick q for the leckies out there!
I have a split load cu with upstairs and downstairs rings covered by common
RCD.
If I turn off the downstairs ring at the breaker and check Live-earth and
Neutral-earth with a crude tester, I get no indication of voltage flow.
If I accidentally dab live or neutral to earth, then it knocks out the RCD.
So where is the current flow being generated which will trip the RCD when
the live is isolated at the breaker?

TIA

Phil

RCD - Why does this Happen?
 

TheScullster wrote:
Hi all

Quick q for the leckies out there!
I have a split load cu with upstairs and downstairs rings covered by common
RCD.
If I turn off the downstairs ring at the breaker and check Live-earth and
Neutral-earth with a crude tester, I get no indication of voltage flow.
If I accidentally dab live or neutral to earth, then it knocks out the RCD.
So where is the current flow being generated which will trip the RCD when
the live is isolated at the breaker?

TIA

Phil


Switching off the MCB will *not* isolate the neutral


If the house neutral is even at only a few mV with respect to earth -
and it usually is - then any short will cause the RCD to trip. Indeed,
this is one of the more common causes of RCD misbehaviour.

RCD - Why does this Happen?
 

If I turn off the downstairs ring at the breaker and check Live-earth and
Neutral-earth with a crude tester, I get no indication of voltage flow.

No such thing as voltage flow. Step away from the crude tester. Someone
might get hurt.

If I accidentally dab live or neutral to earth, then it knocks out the
RCD.
So where is the current flow being generated which will trip the RCD when
the live is isolated at the breaker?

It goes from the live of the "on" circuit, through an appliance to neutral
on the "on" circuit. This is connected directly to the neutral of the "off"
circuit, through an appliance on the "off" circuit into the live of the
"off" circuit and into the earth of the "off" circuit and into the ground.
There are two appliances worth of resistance, but it can be enough to trip
an RCD.

It is more likely on a neutralearth leak on the "off" circuit, as there is
no need to get through an appliance to get to the shorted live.

Christian.

RCD - Why does this Happen?
 

"Christian McArdle" wrote

No such thing as voltage flow. Step away from the crude tester. Someone
might get hurt.

It goes from the live of the "on" circuit, through an appliance to neutral
on the "on" circuit. This is connected directly to the neutral of the
"off"
............snip................

Thanks Christian

Just to put your mind at rest, the tester is a pukka mains voltage device
with LED voltage indicators and self test feature.
The word "crude" was used, perhaps incorrectly, to differentiate this from a
high spec multi-meter.

Phil

RCD - Why does this Happen?
 

TheScullster wrote:

Quick q for the leckies out there!
I have a split load cu with upstairs and downstairs rings covered by common
RCD.
If I turn off the downstairs ring at the breaker and check Live-earth and
Neutral-earth with a crude tester, I get no indication of voltage flow.

If you were to check with a decent multimeter you would probably see a
small voltage difference between your neutral and earth. Especially if
you have a TN-S earth (i.e. typically provided via the sheath of the
incoming cable).

If I accidentally dab live or neutral to earth, then it knocks out the RCD.

To be expected - more so with the neutral to earth connection.

So where is the current flow being generated which will trip the RCD when
the live is isolated at the breaker?

A voltage present on the neutral can also find a route through a
connected appliance to the live when the MCB is open.

--
Cheers,

John.

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RCD - Why does this Happen?
 

TheScullster wrote:
Hi all

Quick q for the leckies out there!
I have a split load cu with upstairs and downstairs rings covered by common
RCD.
If I turn off the downstairs ring at the breaker and check Live-earth and
Neutral-earth with a crude tester, I get no indication of voltage flow.
If I accidentally dab live or neutral to earth, then it knocks out the RCD.
So where is the current flow being generated which will trip the RCD when
the live is isolated at the breaker?

TIA

Phil

Check the archives (e.g. groups.google.com) before posting as this is a
FAQ (even if it hasn't made the formal list)... :-)

Mathew