OT - Fitting RCD in household mains supply

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Sparks wrote:

You haven't thought it through. there is ALWAYS some neutral earth leakage. It occurs every time an oh so rigorously RFI suppressed switch mode power supply (TV, computer, monitor, LV lighting) or any suppresed motor is plugged into the circuits. Enough of these and the 30mA RCD is on a hair trigger. Any voltage surge and over she goes.
Here it was a toss up whether the RCD. MCB, or both would go.
Now with a 100mA RCD, its not gone since.
Whether or not I install RCBO's in the few circuits the regs demand depends on the building inspector.
Proper standrads of wiring and better plugs, sockets, and particularly the use of double insulation and plastic cases have done far far more to reduce shock hazard than RCD's.
I can clearly remember getting shocks of metal fires, whose earthing wwas poor or nonexistent, and whose rubber coated wires frayed all too easily. Now we seldom have metal frames, and we don't have users installing their own plugs nuch either.

I have just explained why.

Actually, I think its quite good, provided it trips only when there is a significant dangerous fault.
Its the nusiance tripping that is so serious.

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the
when
Where does this stray energy go? All LV Transformers I have seen only have L&N, no earth, so where does it go? Into capacitors??

What use is a 100mA RCD?
You die with a 40mA shock, that's why they make them 30mA

Just like a split load would, but it would only take out the faulty section, not the whole lot. (If it took out the lighing, then there are still table lamps!)

Well I have NEVER had one single nuisance trip on my lighting RCD, and it has been in since the 1980's It has never tripped when a bulb has blown - the only time it did trip was when some dickhead drilled through a cable (A builder, NOT me!!)
We did however have a fault develop with our "sockets" RCD, this was then tested (along with the lighting one) and it was found to be defective (over sensitive)
Sparks...
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Sparks wrote:

Mmm. Ok, I'll take out the LV transformers. Bercause thinking about mine, they are alos two wrire devices. That is probably why they interfere so badly with radios.

Well it trips if you get a neutral ertha short, or some other appliance problem.
Whether you die at 40mA is not an established yes and no type thing.
Much less applied across the heart can stop it in a vulnerable subject: Much more applied across just a finger, will burn, but not kill.
The problem with 30mA is its proneness to false triggering, especially applied to a whole house.
The other issue is WHAT should be protected. Arguably the most dangerous circuit in the house is lighting, where you have potentially two user accesible terminals, and a (now rigorously earthed) metal housing in close proximity. Grab the housing with one hand, and stck yer other finger in the socket and Whazza! you are across the floor...

Not in my house. They have theor own lighting sockets on the relevant lighting circits.
I like the idea of split loads, but Peter has made me rethink all of this.
Frankly, I cannot see an ideal solution.
I am very happy that the discussion continue - if a general consensus of best practice CAN be established, then a fait accompli could be presented to those who dream up building regs.

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Firstly, it usually takes about 200mA to be killed. Secondly, a 100mA RCD is not for supplementary direct protection. i.e. it is not intended to prevent electrocution when you touch the live. Its purpose is to rapidly disconnect the supply in the event of a minor earth fault, rather than relying on there being a massive earth fault. Circuits likely to require supplementary direct protection should use dedicated 30mA RCBOs for that circuit only. Whole house RCDs (except 100mA Type S) and even split load consumer units are a bodge.
Christian.
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wrote:

Mostly - increasingly switched mode PSU's require an earth (and noise filter) to meet the EMC directive. There is some discussion going on now as to whether house circuits should be classified as high protective conductor current circuits (which would effectively stop 30mA RCD's being fitted to these circuits).

No you don't, it's nothing like that simple. The chances of death from a brief shock caused by touching a conductor carrying below several thousand volts is quite low. The figures vary depending upon the conduction path - the most dangerous being through your left arm to the feet or right arm (hence the advice to keep your left hand in your pocket when working on live circuits). Whether you grasp the conductor is also critically important, unless you grasp the conductor 240V is rarely fatal (always use the back of your hand for first contact if you really have to touch something which may be live). Even small currents cause the hand muscles to contract preventing you from letting go.
There are widely varying figures depending upon where you look for the simple reason that not a lot of practical work has actually been carried out on the subject. For some reason they couldn't get many volunteers. Most of the levels are therefore extrapolations from the results seen at lower levels.
Critically important is the "Let go" current, the current above which you cannot release your grasp upon a live conductor. 99% of the female population have a “let go” limit above 6 mA, with an average of 10.5 mA. 99% of the male population have a “let go” limit above 9mA, with an average of 15.5mA.
Prolonged exposure to 50 Hz currents greater than 20mA across the chest causes the diaphragm to contract which prevents breathing and causes the victim to suffocate so it is quite possible (albeit improbable) to die while your RCD stays firmly in place.
The more immediate cardiac effects depend upon the frequency of the electrical current as well as its magnitude. Humans are most susceptible to frequencies at 50 to 60Hz as the internal frequency of the nerve signals controlling the heart is approximately 60 hertz. Ventricular fibrillation occurs when 50/60Hz current from the electric shock interferes with the natural rhythm of the heart. At 100mA the current would need to be maintained for about 3 seconds to start fibrillation, at 900mA it would take only 3 milliseconds. The heart loses its ability to pump and death quickly follows.
Ventricular fibrillation can occur at current levels as low as 30 mA for a two year old child and 60 mA for adults. Most adults will go into ventricular fibrillation at hand to hand currents of about 100 - 200 mA.
Humans are able to withstand 10 times more current at DC and at 1kHz hertz than at 50 or 60 Hz. Interestingly the most dangerous current range is somewhere between 200mA and 4A which causes the heart to fibrillate which cannot be stopped by first aid methods. Above that current the heart is paralysed rather than going into fibrillation and simply stops - it can sometimes be restarted with a blow to the chest.
The following figures are taken from NASA, Stanford and the Royal Navy.
Current Physiological reaction <1 mA None 1 mA Perception threshold 1-3 mA Mild sensation 3-10 mA Painful sensation 10 mA Paralysis threshold of arms 30 mA Respiratory paralysis 75 mA Fibrillation threshold (0.5%) 250 mA Fibrillation threshold (99.5%) 4 A Heart paralysis threshold >5 A Tissue burning
Sub lethal electrical shocks can often cause permanent nerve damage which only becomes apparent weeks or months later when the wastage of muscle tissue is noticed.
50-150 Milliamperes - Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.
0.2-4A - Ventricular fibrillation. (The rhythmic pumping action of the heart ceases.) Muscular contraction and nerve damage occur. Death is most likely.
10A - Cardiac arrest, severe burns and probable death.
Under 1 mA - Not perceptible 1 to 8 mA - Mild sensation 8 to 15 mA - Painful shock but person still has control of muscles and can let go of source of shock 15 to 20 mA - Painful shock. Unable to let go 20 to 50 mA - Severe shock. Severe muscular contractions. Breathing difficult 50 to 100 mA - Extreme shock. Extreme breathing difficulties 100 to 200 mA - Death by Ventricular Fibrillation. Not reversible 200 mA up - Severe burns. Breathing stops. Chest muscles clamp and stop the heart but no ventricular fibrillation. Victim may survive if given immediate resuscitation.
Assuming the skin (which is quite a fair insulator)is intact typical resistance levels for an adult are:-
Dry skin 100k to 600k Ohms
Wet skin - 500 to 1k Ohms
Internal (broken skin) hand to foot - 400 to 600 Ohms, Ear to ear ~100 Ohms.
Unless you have wet hands it much more difficult than you might think to dangerously electrocute yourself with 240VAC. Certainly in your changing a light bulb scenario it would be most improbable.

A 30mA RCD will pass its test if it trips at 16mA.
--
Peter Parry.
http://www.wpp.ltd.uk/
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But it is all a lot more clear now Peter.
Could you possibly tie in with our FAQ chief to lodge the essence of what has really been a multi-stage tutorial?
It would be much more readily accessible to future enquirers in the FAQ, than Googling thro a 42 element thread.
How do you feel about 100mA rcd protection of lighting circuits as a fire-prevention measure? I have a special interest in this, having a house where rodents have been chewing at the wiring.....
--
roger

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writes

Good idea about the FAQ.
you mention fire prevention & 100ma RCDs
In the US they tackled this a different way with AFCIs - Arc fault interuptors see here http://members.tripod.com/~masterslic/afci.html
Should we go this way too ?
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I have this problem at work and have to use RCD 13A outlets which is expensive but at under £10 per plug not as bad as it used to be
snipped some really interesting stuff
very informative. Ta
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wrote:

Of course there can be, and is. You are forgetting what happens when a bulb goes and the effects of inductance. You are also forgetting the effect of sensitisation by unbalanced noise filters. Indeed it is questionable if simple 30mA protected power circuits have any future because of this alone.

Perfectly serviceable.

Because their most common failure mode is fail on power on and they have poor reliability.

CFL, permanent low level lighting, whatever suits the need.

Any RCD on the lighting circuit is bad, and it is that which you do appear to be suggesting.
--
Peter Parry.
http://www.wpp.ltd.uk/
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This is not my experience either.
Can you propose a mechanism for this? The only one which comes to mind would be that you already had a neutral-earth leakage fault, and the rise in neutral current caused by the bulb-failure arc was sufficient to create a leak to earth.

It wouldn't matter what kind of lamp illuminated the stairs if the lighting circuit had tripped.

Yes, but you're a (former?) member of the emergency services, aren't you? As such, I have every sympathy with the gruesome sights you might have had to deal with in your chosen profession, but I do believe that members of such services are uniquely *ill* placed to give general advice about risks to ordinary people.
To a fireman, a horrible death in a house fire is a common occurence - to me, as a resident of a non-smoking household, it's extremely unlikely to be the cause of my, or my family's demise. Personally, if I'm doing the neurotic parent bit I worry about road safety a darn sight more.

TBH, if you know of a property that has an RCD which trips when light bulbs blow, then you ought to advice the occupants to get the wiring fixed.
Will
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Will Dean wrote:

There is always some leakage from both L & N to earth. If nothing else due to the capacitance between the miles of T & E laid over the house. And more especially in all the capacitors used to RFI filter electronic devices.
There is always some inductance in the supply, due to cable lengths
and transformer secondaries. High speed switching currents will
work with the capacitances to flip RCDs
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On Tue, 16 Dec 2003 20:29:09 -0000, "Will Dean"

You don't need resistive leakage to trip an RCD and the problems of pre-sensitisation are become more acute as more equipment includes noise filters.

Indeed not, that is why you chose a lighting type of high reliability whose failure mode is such that it would not trip protective circuits. Incandescent bulbs have poor reliability and less than optimal failure characteristics.

No.
Why? Because they understand the balance of risks better?

Actually it isn't.

Quite reasonable, but risk assessment is about how much work you have to put into circumventing a risk. Removing (or not fitting) an RCD on a lighting circuit is trivial and although the risk of fire is small the consequences are devastating and I suggest the minimal effort is worth it.

What's to fix? I can set up a demonstration to prove an RCD will trip on a bulb blowing on a perfectly serviceable circuit with ease. The point is that an RCD on a lighting circuit does no good at all but potentially may be very harmful, a situation recognised in the wiring regulations. What's the agrement for fitting them?
--
Peter Parry.
http://www.wpp.ltd.uk/
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in
I dont know enough about this to comment, only that as I have said before, in my installation (With incadecent, CFL and LV switchmodes) I have never had the RCD trip on a bulb failure (Including with 500w halogens) - So in my setup I really dont see the added risk. please explain why in my setup it wouldnt be...

bulbs
Sorry but I totally disagree with that statment. It will stop people getting killed by electrocution.

Please explain, in a property where there are never any nucence tips, whay is it bad?
Sparks...
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rise
a
wouldnt
I'm quite prepared to be shot down in flames BUT I operate a wet area supplied via 30ma RCDs ..it has 3kw of halogen lighting - failure mode is water spray on tubes. it also has two water pumps many switchmodes many fractional HP motors a large compressor many incandescents several EMI filters many 240v 3 port valves which are damp and the only RCD trip we ever had was from a cable rupture in a pudle of water. MCBs pop often with incandescant failure but never the RCD (it's NICIEC tested regularly)
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On Wed, 17 Dec 2003 01:32:03 -0000, "Chris Oates" <none> wrote:

No shooting, but was is it?
.andy
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A shooting gallery ;)
Hillbilly Moonshine
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wrote:

People don't get killed by electrocution from lighting circuits. They may sometimes get a belt from them (which an RCD will reduce but won't prevent) but they do not cause death or injury.

Because in a fire the lights all go out as the RCD trips.
Electrocution in the home kills 25 people a year and causes 2000 minor and serious injuries. I only have access to detailed figures for 1998 and in that year none of the 23 deaths by electrocution involved lighting circuits - all bar 6 were people dismantling live equipment, the remaining 6 were handling faulty supply cords.
In one year 500 people die and 18,000 are seriously injured in fires in the home.
That's why the wiring regulations don't support fitting of RCDs to lighting circuits.
--
Peter Parry.
http://www.wpp.ltd.uk/
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JOOI, can you describe the circuit? As I say, I have many years experience of non-nuisance-tripping 30mA whole-house RCDs, and always with a room full of computer equipment somewhere on the circit.

I'm not particularly arguing for the fitting of them against the regulations, only against the received wisdom that A. they'll give you lots of nuisance tripping, and B. that they're the major cause of nuisance tripping on lighting circuits. (Which we can all agree is a bad thing.)
However, my experience of whole-house RCDs is that they've given warning of incipient problems, both with wiring and with appliances. For a start, they detect neutral-earth failures (which one would guess represent about 50% of insulation problems) which are not typically detected by overcurrent protection at all. They also give early warning of thinks like heater sheath failure in immersion heaters and electric showers - I don't consider myself odd in thinking that I'd like to know about an immersed heater failure while there's 50mA flowing to earth rather than 50A.
And, like I say, I know someone personally who had a bad shock off a fixed light fitting, and a teenage girl in the village I went to school in was killed by an electric shock from a light fitting - her parents returned to the house after a holiday to find her dead on the living room floor.
I'm afraid I interpret your gruesome anecdote as a warning about the way one locks ones house at night (not being clear that it has any link to lighting circuit RCDs, anyway), and mine as warnings about earth protection on wiring . No doubt we both feel rather closer to our own cautionary tales.
Will
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Well said Peter. You are probably a voice in the wilderness, like that chief inspector who categorically stated that 'excess speed is the cause of less than 7% of accidents' and 'speed cameras do not (statistically) reduce road deaths'.
I am definitely with you on this. My 30mA RCD tripped about 50% of the time when any bulb blew, with the MCB about 60%. Sometimes both.
Its bloody dangerous feeling down te saircase to get to the consumer unit, especialy if you have left the casing off...:-)
Peter Parry wrote:

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wrote:

Reading this, and especially the very sad ending to Peter's post makes me think that it would not be a bad idea to add emergency lighting as something that should be included in Building Regulations for new properties and incentivised for others in some way, especially for the elderly and others with mobility difficulties..
We already have mains powered smoke detectors as an addition, basically to get over people taking the batteries out or forgetting to replace them.
Non-maintained small fluorescent lights are not very expensive, although the fittings are pretty ugly. Undoubtedly this could be improved if there were a consumer premises market for them.
It would seem to me that the populace would be better served with a lighting measure like this rather than the mandatory low energy lamps stuff. In fact with a bit of creativity, both objectives could be met in a single package.
I am not sure what sort of size and power level that they would need to be to provide sufficient light say at the top and bottom of a staircase in a fire, but to be able to see adequately during a power cut, then the little 8w ones are probably enough.
I have my consumer unit at the back of a cupboard in the kitchen and it would not be easy to see in the dark. I fitted a small maintained fitting, run from a (non-RCD ptotected) lighting circuit and operated also by a plunger switch on the cupboard door. Thus I have a light over the CU for when I want to work on it with the power deliberately off and also to be able to see the breakers after a trip.
.andy
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