Reverse Polarity Mains Socket -- How Dangerous?

So, in summary...?

snipped-for-privacy@care2.com wrote:

Reply to
Andrew Thelwell
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of cours, yes, you do have to have an earth connection.

NT

Reply to
meow2222

And how is this the fault of the elcb, what mechanism are you proposing? It seems clear you had a faulty installation. I'd rule out a major camera wiring fault as you'd have felt that :)

ELCBs work by sensing the voltage difference between mains earth rod and a separate sense electrode placed elsewhere in the ground. For

30-50v pd to be reached, which is needed to trip an elcb, you have to have a fairly large current flow down the mains earth wire. If touching a camera to anything trips an elcb, there is something very wrong with your setup. The big plus of ELCBs is that of all earth leakage protection devices they are the least liable to nuisance trips. Theyre not perfect, but theyre orders of magnitude less problematic than RCDs.

NT

Reply to
meow2222

Neons have a strike voltage in the region of 80v, so it is not possible for them to light on N-E voltage. The meter swapper was seeing capacitive leakage from mains wiring to the ladder, through him and the screwdriver, to anything he touched. The neon lighting was meaningless in this case.

NT

Reply to
meow2222

My comment is not related to Andrews' electrical problem.

I cannot see Andrews post (above) on my server (Clara) I can see it using Google groups.

AFAIK all the other posts in the thread are visible including Andrews later ones with the photo references, so what is going wrong for me and this particular post of Andrew?

An interesting thread by the way.

Roger

Reply to
Roger R

Hello all, Andy (the OP) here again.

Thanks all for your feedback.

This morning I rewired the socket. It was simply transposed (red wire wired to neutral, black to live). I've swapped them, tested the socket, and all is OK. So that's that one sorted.

I've been looking back over all your posts and been trying to work out what the balance of opinion was with regard to whether or not this may have had something to do with a) the meter man's odd readings and b) the trips on our current-operated ELCBs (RCDs).

Am I right in saying, then, that this particular socket was almost certainly not to blame for the odd readings by the meter man?

Am I also right in saying that his concerns, based on the lighting up of his indicator screwdriver tool, were not necessarily very accurate?

Finally, am I also right in saying that this issue is probably not the root cause of the RCD/ELCB trips?

Sorry if it appears I'm repeating myself, I'm just trying to get a clear summary understanding of all the posts thus far.

Thanks,

Andy

Roger R wrote:

Reply to
Andrew Thelwell

On 28 Nov 2006 07:31:39 -0800 someone who may be "Andrew Thelwell" wrote this:-

Was anything plugged into it at the time he came with his screwdriver? If nothing was then it is extremely unlikely, if something was then there is a slight possibility if it has some sort of electronics in it.

Not very accurate at all, to the point of wondering why he bothered. The real sparks who came later would have found any fault, assuming it was not something intermittent.

Yes. The cause of these needs further investigation.

Reply to
David Hansen

The neon screwdriver indications are worthless, and the conclusions he drew from them incorrect.

The socket wires transposed has nothing to do with the RCD trips you've had.

AFAICS the only problem you've got now is rcd tripping, and thats a commonish problem with rcds. The next move is to insulation test all your appliances with a multimeter.

NT

Reply to
meow2222

One less variable to worry about then...

From the pictures it seems as if you do have a RCD - although a resonably old design.

Yes, I think so.

It is very unsafe to rely on indications from these things. They light up any time you stick enough voltage across them and they pass negligble current. Hence it is easy to get spurious results simply from a wire that is capacitively coupled to another one, or worse, from your body being coupled to something live by the same mechanism. This can result in you appearing live to the base of the screwdriver. So touch the tip to a live thing (i.e. same potential as you) and it does not light, touch it to a neutral conductor and it does. Get it so that you are hanging about with a coupled or induced voltage of about 100V and everything looks live!

Yup. You have either faulty wiring (unlikely unless it is old rubber stuff) or more likely one or more faulty appliances.

Reply to
John Rumm

Well that does rather depend >> ... one of those 'mains tester' screwdrivers with the LEDs in the

I assumed it was a 'voltstick' since LEDs were mentioned, and the uninitiated might not realise that it wasn't a screwdriver and didn't require contact. A voltstick, as most readers will know, will light up when a butterfly flaps its wings in New Zealand.

I'd be very surprised indeed if meter fitters were being issued with neon screwdrivers.

Reply to
Andy Wade

I've never known that type of device to be fitted in a house. The usual type of old ELCB (typified by that black Crabtree thing) had a low impedance coil and was wired in series with the earthing conductor - i.e. between the main earth terminal and the earth electrode. They were rated in terms of a tripping current, 30 mA, and the winding impedance was such that the coil voltage drop to trip was around 6 V. Their major disadvantages were (a) that the coil could easily become shorted (or partially shunted) by parallel earth paths, causing loss of protection, and (b) that they could (and would) suffer from nuisance trips from faults outside the installation - e.g. as a result of overlapping earth resistance areas with neighbouring premises.

Example of problem (b) above, perhaps.

That's just complete and utter bollocks. RCDs are an order of magnitude more reliable than old ELCBs and have the advantage of providing shock protection against direct as well as indirect contact.

Reply to
Andy Wade

Andy, sorry but I disagree. I have never seen a voltage operated ELCB which has an earth current rating, as you suggest. I have a couple (removed from homes and light commercial premises), and have seen many still in use. Of course, since the coil has a resistance, ohms law will dictate that there is a tripping current as well as a tripping voltage, but that's not how the devices are intended to work. They don't expect to see all the current associated with any earth leakage in the first place, so a current rating makes no sense.

No, this doesn't cause loss loss of protection (unless the coil sense electrode has an overlapping earth resistance area). You can have as many parallel earth paths as you like (see diagram in 14th edition regs). If you have enough, the ELCB will stop working, but this is because it has ceased to be necessary, and is not an error situation (it is explicitly allowed for in the testing procedure). In this case, and earth leakage will either trip the fault current protection or remain safe (CPC not raised as much as 50V above real earth).

Yes, that was a big problem.

Agree completely.

Reply to
Andrew Gabriel

You illustrate the first problem with elcbs, namely that there is more than one type of device referred to as an ELCB. There are current operated ones and there are voltage operated ones. Your comments apply to current devices, mine to voltage devices.

Voltage operated devices are much more reliable for one simple reason, they dont trip until enough earth current flows to cause a considerable rise in earth potential. This wipes out the majority of nuisance trips. Current operated devices are quite a different can of worms, and I dont have much to say about those.

NT

Reply to
meow2222

That's no surprise - we've been here before...

ICBW, but I have a fairly clear memory of these being sold as "30mA earth trips" and similar descriptions in the pre-RCD era. Anyway, after a bit of a rummage in the TQT pile I found a Crabtree one this afternoon, shown here with the terminal covers removed:

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's no external marking of any trip rating, but the lower terminal cover carries a moulded legend thus:

For max. earth res of 500 ohm Impedance of E. L. trip coil 500 ohm Max tripping current 35 mA

Image here:

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's no British Standard no., and certainly no indication of any trip voltage rating/characteristic.

Well that's where we may have to agree to differ.

I can see your argument, for a device working in the way you claim, but if the ELCB didn't operate then an earth fault would not necessarily be cleared, since the current will be limited by the earth resistance at the supply end (distribution transformer earthing) and might not be sufficient to cause an OPD to operate. The local earth voltage would not rise sufficiently to cause danger from shock, but there's a risk of danger from thermal effects and of shock to other consumers on the network, since the supply neutral will have been pulled well away from earth.

What test procedure are you referring to above? An earth fault must be cleared somehow, and in a TT system you can't rely on getting enough current to operate an OPD, so the ELCB or RCD simply _must_ operate within the permitted disconnection time.

I don't have a copy of the 14th edition, but ELCBs were still permitted during the early part of the life of the 15th.

Here are the relevant regs from the 15th ed. (OCR'd). I see nothing to suggest any requirement of a separate sensing earth electrode:

413-6 Where compliance with the disconnection times of Regulation 413-4 is afforded by a residual current device in an installation which is part of a TN or TT system, the product of the rated residual operating current amperes and the earth fault loop impedance in ohms shall not exceed 50. If a fault-voltage operated device is used in a TT or IT system the earth fault loop impedance, including the earth electrode resistance, shall not exceed 500 ohms.

NOTE - For TT systems the earth fault loop impedance includes the earth electrode resistance.

413-11 Where protection is afforded by fault-voltage operated protective devices, all exposed conductive parts and associated extraneous conductive parts protected by any one such protective device shall be connected by protective conductors to an earth electrode via the voltage-sensitive element of that device. 413-12 The protective devices shall be of one or more of the following types: - residual current devices, - overcurrent protective devices, - fault-voltage operated protective devices.

NOTE - Residual current devices are preferred.

471-15 Automatic disconnection using fault voltage operated protective devices is recognised for use in TT and IT systems and is suitable where the impedance of the earth fault loop prevents compliance with Regulations 413-4 and 413-5 by the use of overcurrent protective devices.

NOTE - The use of residual current devices is preferred.

531-9 The characteristics of every fault-voltage operated protective device shall be such as to comply with Regulation 413-3 for automatic disconnection in the event of a fault of negligible impedance between a phase conductor and exposed conductive parts, taking into account the impedances of the fault current loop at every point at which this method of protection is to be applied. For TT systems, allowances shall be made for any likely increase in the value of earthing resistance with time, for example, with seasonal variations (see also Section 544). 544 EARTHING, AND PROTECTIVE CONDUCTORS, FOR FAULT-VOLTAGE OPERATED PROTECTIVE DEVICES 544-1 Where protection for safety is afforded by a fault-voltage operated protective device, the requirements of Regulations 544-2 to 544-5 shall be observed. 544-2 An independent earth electrode shall be provided outside the resistance area of any other parallel earth. If by sub-division of the earthing system discrimination in operation between a number of fault-voltage operated protective devices is to be afforded, the resistance areas of the associated earth electrodes shall not overlap. 544-3 The voltage-sensitive element of the protective device shall be connected between the main earthing terminal and the earthing conductor. 544-4 The earthing conductor shall be insulated to avoid contact with other protective conductors or any exposed conductive parts or extraneous conductive parts, so as to prevent the voltage- sensitive element from being inadvertently bridged. 544-5 Protective conductors shall be connected only to the exposed conductive parts of those items of equipment whose supply is to be interrupted in the event of operation of the fault-voltage operated protective device, and to any extraneous conductive parts in the same equipotential zone. 613-16 Where protection against indirect contact is to be provided by a residual current device or a fault voltage operated protective device, its effectiveness shall be verified by a test simulating an appropriate fault condition and independent of any test facility incorporated in the device (see Item 6 of Appendix 15). [Testing] 6.2 Operation of fault-voltage operated protective devices

A test voltage not exceeding 50V r.m.s. a.c., obtained from a double wound transformer connected to the mains supply, is applied across the neutral and frame terminals and the device shall trip instantaneously. The transformer has a short-time rating not less than 750VA (see Figure 19).

Figure 19 (scanned) is here:

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sample Crabtree device trips at a shade over 40 mA RMS, at which current the voltage across the coil is about 11.5 V. The coil impedance is thus under 300 ohms (its DC resistance is about 160 ohms). When tripped (or just switched off) the coil is shorted out (by the contact pair shown under the coil symbol in the legend photo).

At this point I'll rest my case and let you ponder...

Reply to
Andy Wade

Hello all, OP here...

Yes, it may not have been a screwdriver at all, it just looked a bit like one, and it certainly wasn't a neon bulb in the end... it had (more than one) small red LEDs fitted into the handle, as I remember...

Don't know if that makes any difference?

Regards,

Andy

Andy Wade wrote:

Reply to
Andrew Thelwell

A Current Operated ELCB is the original name for an RCD (14th Ed wiring regs).

Reply to
Andrew Gabriel

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Don't know if that makes any difference?

Well, they are safer to use in the sense they are non contact - no need to go exposing live parts just to touch them. You can still get silly results with them though.

Reply to
John Rumm

Something like this?

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Reply to
Dave Plowman (News)

Hi Dave,

No -- it was a 'single stick' unit... like a screwdriver or pen type appearance. My memory says it looked more like a screwdriver but it wasn't a screwdriver per se.

Something more like the voltstick, yes...

Regards,

Andy

Dave Plowman (News) wrote:

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Reply to
Andrew Thelwell

Yes, something broadly like that.

Dave Plowman (News) wrote:

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Reply to
Andrew Thelwell

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