Following a thread on Sagazone (just one funny allowed!) where someone
has a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the
earth leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."

Richard

"Dickie mint" wrote in message
...
Following a thread on Sagazone (just one funny allowed!) where someone has
a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the earth
leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."


The usual method of attaching a leaky device is to use an isolating
transformer (the output of which also floats thus halving the voltage to
ground).

--
Bob Mannix
(anti-spam is as easy as 1-2-3 - not)

On Aug 18, 1:35*pm, "Bob Mannix" wrote:
"Dickie mint" wrote in message
...

Following a thread on Sagazone (just one funny allowed!) where someone has
a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the earth
leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."

The usual method of attaching a leaky device is to use an isolating
transformer (the output of which also floats thus halving the voltage to
ground).

It works and it gets rid of the shock risk, mostly, but there is still
capacitive leakage in the transformer and still the possiblity of a
2nd leak on the load which could again expose a shock risk. So yes it
works, but its not best practice.

Iso tranformers aren't 'coming onto the market' they've been around a
century or so.

Finding and fixing the fault would seem a more sensible approach. Only
if fixing wasnt practical and the item were of high value would I ever
consider an alternative protection system involving an iso tf.


NT

wrote:
On Aug 18, 1:35 pm, "Bob Mannix" wrote:
"Dickie mint" wrote in message
...

Following a thread on Sagazone (just one funny allowed!) where someone has
a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the earth
leakage outing the RCD"
Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?
The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."
The usual method of attaching a leaky device is to use an isolating
transformer (the output of which also floats thus halving the voltage to
ground).

It works and it gets rid of the shock risk, mostly, but there is still
capacitive leakage in the transformer and still the possiblity of a
2nd leak on the load which could again expose a shock risk. So yes it
works, but its not best practice.

Iso tranformers aren't 'coming onto the market' they've been around a
century or so.

Finding and fixing the fault would seem a more sensible approach. Only
if fixing wasnt practical and the item were of high value would I ever
consider an alternative protection system involving an iso tf.


NT
The trouble is in many cases there is no fault: Just a lot of RFI style
filters and cable capacitance, that, combined with any spike on the
line, has enough surge to blow an RCD.

On 18 Aug, 14:59, wrote:

Iso tranformers aren't 'coming onto the market' they've been around a
century or so.

"Century or so" sounds surprisingly long. Just how old is the first
isolating transformer (as a portable device, for the use of portable
appliances) and what was it used for?

In article ,
Dickie mint wrote:
Following a thread on Sagazone (just one funny allowed!) where someone
has a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the
earth leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."

I'd be surprised if this happens in any domestic installation.
For a pro installation they should be suitable radial circuits with likely
loads calculated. Rings are for domestic use where these things (load etc)
vary somewhat - and aren't known at installation time. But so saying most
will use a radial for utility room appliances since these too can cause
problems with RCDs.

--
Small asylum seeker wanted as mud flap, must be flexible and willing to travel

Dave Plowman London SW
To e-mail, change noise into sound.

On Aug 18, 3:32*pm, Andy Dingley wrote:
On 18 Aug, 14:59, wrote:

Iso tranformers aren't 'coming onto the market' they've been around a
century or so.

"Century or so" sounds surprisingly long. Just how old is the first
isolating transformer (as a portable device, for the use of portable
appliances) and what was it used for?

Late 1800s IIRC. I think they were first used for mains power
distribution.


NT

In article ,
Dickie mint writes:
Following a thread on Sagazone (just one funny allowed!) where someone
has a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the
earth leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."

Max leakage for a piece of Class I (earthed) IT equipment is 0.75mA
You would need something like 40 pieces of Class I (earthed) IT
equipment to get near 30mA leakage, assuming they all leak the
max permitted. More likely, you have one faulty one.

You should design circuits with a leakage of no more than 25% of
the RCD trip rating. For a 30mA RCD, that's a design leakage of
7.5mA. That's a maximum of 10 Class I (earthed) IT appliances
per RCD. In conventional computer terms, that would be 5 PC base
units and 5 monitors. Some LCD monitors are Class II double
insulated nowadays, and in that case you could get away with 10
PC base units. Most other small computer peripherals are also
Class II double insulated and can be ignored, but larger ones
are often Class I (earthed) and need to be allowed for.

With increasing use of IT/entertainment equipment in the home
and increasing use of RCD protected circuits, provision of more
separately RCD protected socket outlet circuits should be
considered. Hanging them all off one RCD is asking for trouble.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]

Andrew Gabriel wrote:
In article ,
Dickie mint writes:
Following a thread on Sagazone (just one funny allowed!) where someone
has a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the
earth leakage outing the RCD"

Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?

The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."

Max leakage for a piece of Class I (earthed) IT equipment is 0.75mA
You would need something like 40 pieces of Class I (earthed) IT
equipment to get near 30mA leakage, assuming they all leak the
max permitted. More likely, you have one faulty one.

Is that DC or AC?

By my reckoning, 10nF of live to earth capacitance will make that figure
at nice clean 50Hz sinewave. Now shove a spike on it with loads of
harmonics..

In a typical house installation there is a huge amount of inter-wire
capacitance. Every RFI filter (mandatory on just about any piece of
electronics including CFL light bulbs) probbaly adds a few nF or more..




You should design circuits with a leakage of no more than 25% of
the RCD trip rating. For a 30mA RCD, that's a design leakage of
7.5mA. That's a maximum of 10 Class I (earthed) IT appliances
per RCD. In conventional computer terms, that would be 5 PC base
units and 5 monitors. Some LCD monitors are Class II double
insulated nowadays, and in that case you could get away with 10
PC base units. Most other small computer peripherals are also
Class II double insulated and can be ignored, but larger ones
are often Class I (earthed) and need to be allowed for.

With increasing use of IT/entertainment equipment in the home
and increasing use of RCD protected circuits, provision of more
separately RCD protected socket outlet circuits should be
considered. Hanging them all off one RCD is asking for trouble.

Oh,indeed.

However one wants that 'last line of defence' of an overall RCD..and
RCBO's are bulky and expensive.

Anyway, my point was that RCD's will trip with no fault as such other
than a lot of RFI capacitors across the mains.

On Aug 18, 3:04*pm, The Natural Philosopher wrote:
wrote:
On Aug 18, 1:35 pm, "Bob Mannix" wrote:
"Dickie mint" wrote in message
...

Following a thread on Sagazone (just one funny allowed!) where someone has
a ring main RCd trip out when switchong off or pulling the plug on a
computer distribution pattress. their electrician was quoted as saying :-
"there's something coming on the market to plug into to prevent the earth
leakage outing the RCD"
Apart from horrifying me that something might be available to thwart the
very reason for having an RCD, what is he on about?
The same electrician is reported as saying "when he tested the computer
and plug, the computer went to near the end of 30mA."
The usual method of attaching a leaky device is to use an isolating
transformer (the output of which also floats thus halving the voltage to
ground).

It works and it gets rid of the shock risk, mostly, but there is still
capacitive leakage in the transformer and still the possiblity of a
2nd leak on the load which could again expose a shock risk. So yes it
works, but its not best practice.

Iso tranformers aren't 'coming onto the market' they've been around a
century or so.

Finding and fixing the fault would seem a more sensible approach. Only
if fixing wasnt practical and the item were of high value would I ever
consider an alternative protection system involving an iso tf.

NT

The trouble is in many cases there is no fault: Just a lot of RFI style
filters and cable capacitance

dredges up remants of Electrical Engineering course

Ah yes, Capacitive coupling. I always assumed that it was an
inductance that made the earth wire voltage float upwards. Capacitive
makes a lot more sense.

P.

The Natural Philosopher wrote:

Andrew Gabriel wrote:
Max leakage for a piece of Class I (earthed) IT equipment is 0.75mA
You would need something like 40 pieces of Class I (earthed) IT
equipment to get near 30mA leakage, assuming they all leak the
max permitted. More likely, you have one faulty one.

By my reckoning, 10nF of live to earth capacitance will make that figure
at nice clean 50Hz sinewave.

Yes.

Now shove a spike on it with loads of harmonics..

RCDs will ignore spikes of short duration (fast transients). THD on the
mains voltage waveform is typically only 2 or 3%, despite its horrible
flat-topped appearance on a 'scope, and most of it is in low-order
harmonics - mainly 3rd and 5th - so I don't think harmonics are
particularly germane to the subject of RCD nuisance tripping.

In a typical house installation there is a huge amount of inter-wire
capacitance.

It might be huge to an RF engineer, but is hardly likely to be that
significant at 50 Hz. OOI I've just measured the L-E capacitance of a
metre of 2.5 T&E and its about 80 pF (with the neutral wire guarded
out). Call it 100 pF/m. There's probably something like 300 m of cable
in a typical house, so 30 nF total capacitance and about 2 mA leakage -
and that shouldn't all be going via a single RCD.

Every RFI filter [...] including CFL light bulbs) [...]

Bzzt. All CFLs are Class 2. There's no earth connection, and no earth
leakage unless you want to worry about a few nA through the stray
capacitance to earth.

[...] probbaly adds a few nF or more..

Seldom more than 4.7 nF for the Y-capacitors - less than 0.4 mA leakage
per filter

Anyway, my point was that RCD's will trip with no fault as such other
than a lot of RFI capacitors across the mains.

Obviously /in extremis/ that could happen. IME though it doesn't and
nuisance tripping is invariably caused by some sort of fault condition.

I wonder whether any of the electronic RCDs use phase-sensitive
detection and only respond to in-phase leakage?

--
Andy