I'm installing a bathroom heater and, for maximum
protection, would prefer to connect its power circuit spur
via an RCBO rather than via the more usual switched fused
double pole isolator. The power circuit is still fed from a
fused consumer unit with a "whole house" RCCB -- not
something I'm planning on changing soon.
Anybody aware of an RCBO (= combined RCCB and MCB) that
comes in a single wall box sized package? Heater is rated
at 750 W so looking around 5 Amps trip current.
Or is my difficulty in finding a suitable product because
there isn't enough demand -- showing that I'm thinking too
far off the mainstream way of doing things?
On Mon, 12 Jan 2004 19:27:05 -0000, "Charles M Atkinson"
Readily available at most wholesalers, powerbreaker H92 is what you
want or equivalent. 13A RCD combined with switched fused spur. Just
fits on a standard single box. I use them all the time for powering
outside sockets mainly, brilliant things they are too! I would
prefereably go for the powerbreaker because i've had no problems with
them but you could use a cheaper one if you wanted.
Thanks for that, S. Good news!
The PowerBreaker is favourite for being less "industrially"
styled. Specification says "13A max, DP switched". DP?
Does that mean it can be configured to less than 13 Amp
nominal trip current? Have checked Powerbreaker (Greenwood)
website but could not find that info.
"Charles M Atkinson" wrote
| > Readily available at most wholesalers, powerbreaker H92 is
| > what you want or equivalent. 13A RCD combined with switched
| > fused spur.
| The PowerBreaker is favourite for being less "industrially"
| styled. Specification says "13A max, DP switched". DP?
Double Pole - which is what you need.
| Does that mean it can be configured to less than 13 Amp
| nominal trip current?
It's not an RCBO, it's a fused spur + RCD. So you could take the fuse down
However there is no point in fitting a 30mA RCD if you already have a whole
house 30mA RCD - there is no discrimination between the two.
DP means "double pole", which in turn means that when it trips, it disconnects
both the live and the neutral "load" terminals from the supply. This is a
Good Thing (and standard across just about all RCBs).
As to your other question: you've got knickers twisted around two different
things. The "trip current" is the imbalance between wot-goes-out versus
wot-comes-back, and is a nominal 30mA: that is, if a relatively weeny bit
more or less than the full current going out to the load along the live
wire comes back along the neutral, the PowerBreaker will, um, break the
power, on the grounds that if it's not coming back along the neutral, it's
probably going somewhere it really shouldn't (e.g., through you).
Whereas the "13A max" refers to the *total* load current. For your towel
rail - 800W or so, wasn't it? - a 3A or 5A fuse in the PowerBreaker would
be a little more appropriate than the 13A fuse probably supplied, as it'll
give closer protection to the cable than a 13A one would.
Your remaining problem, though, is that if your whole-house RCB is also a
30mA non-time-delayed unit, there's no way of telling which one of them will
trip first in the event of an earth leakage fault on your towel rail. In fact
it's a little more likely that it'll be the whole-house one which will go,
just because there will be small, harmless earth leaks on appliances on the
other circuits (particularly on various appliances with mains suppressors,
such as washing machines, dishenwashen, compteren and similar), which will
"preload" the whole-house RCB closer to the point of tripping than your
towel-rail-dedicated one. If, On The Other Hand, your whole-house RCD is a
100mA time-delayed one, then you're on a Win, as it will reliably "wait" for
the non-delayed one to do its job firstest.
HTH - Stefek
Thanks to all for info. Am learning fast!
Back to original question -- is an RCBO available?
If not then a combined RCD and fuse (13 A changed for 5 A)
is the best available. It still offers better protection
than a traditional switched fused double pole isolator.
Have just checked the "whole house" RCCB and found it's
actually an ELCB with a 100 mA trip current -- marked
"Crabtree SB6000". Given this extra information, is it
useful (= significantly greater protection) to fit a 30 mA
RCD in series with it?
If there wasn't much "base level leakage" then the ELCB
might allow nearly 100 mA through someone before tripping.
Not good. And, if an ELCB measures current in the
supplier's earth conductor (as opposed to a live/neutral
flow imbalance) then someone could be conducting current to
some other earth. Equally not good?
If I do end up with two RCDs (or an ELCB and an RCD) in
series, I don't care which one goes first because I'm not
expecting frequent faults. Is that a sensible view or have
I failed to consider something important?
Getting theoretical: even if the "whole house" was protected
with a 30 mA RCD, I see some benefit in protecting an
individual outlet with another 30 mA RCD -- in case the
"whole house" one failed to operate when it should. Or are
RCDs so reliable that any improvement from using two RCDs in
series is so small that it is insignificant?
the power breaker fused RCD spur is the easiest
thing for you
the crabtree is for fire protection
as above it's for fire protection not electrocution
definition of ELCB
ELCB is a device installed to detect imbalance in currents
flowing between the AC active and neutral lines.
This imbalance current is also known as earth leakage
current. When ELCB detects the imbalance, it trips
and disconnects the AC power supply.
Hence, ELCB protects people from electrocution.
failed miserably - when the crabtree trips the lights go out
no no no no - no more to say
whole house = bad idea
it's called discrimination - when RCDs in series you never
know which one will trip if they are rated similarly
...reliability ? - don't connect a powerbreaker spur backwards
it'll either smoke or won't work properly
most RCDs will operate upside down & I've not yet had one fail
I use Powerbreaker spurs in a commercial environment
....toilet hand driers - perfect solution
The ELCB doesn't actually measure current imbalance directly like an RCD. It
just measures the earth voltage. If it rises, then it cuts the power. It is
not very effective for shock protection and only works if the current flow
is directly to the electrical installation earth. In a bathroom,
supplementary protection may mean that the likely earth path is NOT through
the electrical installation, particularly if no CPC is part of the
supplementary bonding system. An ELCB does not provide sufficient shock
protection for a bathroom, even before you consider the fact that it is
100mA instead of 30mA.
But, this is what happens already. The addition of the RCD outlet will
reduce the chance of the Crabtree device tripping and cutting the lights.
There is more to say.
What should happen is that the Crabtree ELCB should be retired. Assuming it
serves a purpose (i.e. TT earthing, or to fix a dodgy earth loop impedence),
it should be replaced with a 100mA Type S time delayed RCD.
The consumer unit should be upgraded to a modern DIN rail, if necessary.
There are two options:
1. (best) Insulated consumer unit with 100A isolator incomer. Socket
circuits and anything feeding outside or in the bathroom should use RCBOs.
Other circuits should use MCBs. You may be able to replace the 100A isolater
incomer directly with the time delay RCD, removing the need for a separate
2. (cheapest) Insulated split load consumer unit. Socket circuits (and other
high risk) should use MCBs on the RCD side. Other circuits should use MCBs
on the non-RCD side. Again, it may be possible to replace the overall
incomer with the time delay RCD.
The cheaper option is more likely to be subject to nuisance trips. These
trips will also have more serious consequences in cutting power to otherwise
unaffected circuits. However, both solutions should prevent the lights
cutting after an earth fault in the high risk circuits.
I think there's some confusion in terms here.
You are describing an old Voltage Operated ELCB -- they trip when
the difference in voltage between the earth conductors and outside
ground reaches 50V. They don't normally have current ratings like
30mA, 100mA. They aren't intended to protect against electrocution.
An RCD is also an ELCB, but it's a Current Operated ELCB (the term
RCD didn't appear until some time after the devices appeared).
The easiest way to tell which you have is to see how many earth
terminals the device has. A Voltage Operated ELCB has two separate
ones -- one connected to the installation's earthing, and the other
connected to an earth rod (it works by monitoring the voltage
between these). A Current Operated ELCB (RCD) doesn't need any earth
terminals, although some have one to allow it to check for other
faults such as live-neutral reversal, broken neutral, and sometimes
the test button requires it.
Ah. I've never seen a current operated RCD labelled as an ELCB, only old
voltage operated ones. I hadn't realised there had been a period when RCDs
were sold under that name.
In any case, the solution is the same. It is still best to replace the
entire house unit with a 100mA time delayed one, with RCBOs for individual
circuits. However, provided the old RCD works, it isn't quite as urgent. The
main problem is lack of discrimination under an earth fault, causing the
lights to go out.
The wiring regs of that time (14th Edition) calls them Voltage Operated ELCB's
and Current Operated ELCB's. Manufacturers each invented their own names
for Current Operated ELCB's initially, leading to much confusion because
consumers had no idea if all these differently named things were the same
or not (main problem was in the plug-in RCD market). It was a campaign by
Which? and That's Life! that got a common name agreed across the industry, RCD.
The terms RCCB and RCCD still persist a little in the installation business.
Thanks to all -- again. Thank you, especially, for
I'm going to go with Christian's suggestion, bite the bullet
and replace the consumer unit. Was on the list of things to
do, anyway. Little point in adding protection (and spending
money) for the bathroom heater only when that will be
redundant when planned CU upgrade is done.
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