Path to earth through bathroom floor

[JohnW2]

This message I posted yesterday has been totally lost amidst an endless sea of totally off-topic messages (which I imagine most people are ignoring), so I thought I would start a new topic in case anyone has any interest in what I wrote......

So what's the typical conductivity of a damp concrete floor - say in a bathroom ? Is that enough to cause a dangerous current to flow through someone stepping out of the bath/shower with wet bare feet and touching a tap ? In this case I can see an argument that electric U/F heating will increase safety since it will raise the floor potential to the internal false earth level.

It would probably be a pretty low-impedance, possibly pretty dangerous, route to earth. .... However, I would think it must be extremely rare to have a ground floor bathroom with a concrete floor built straight onto earth with no DPM or suchlike, and with absolutely no floor covering. The moment one has virtually any floor covering (or any 'anti-damp' measures in, or under, the concrete), the path to earth probably usually becomes high enough for any current through a person not to be, in itself, particularly dangerous.

OK, I’ve done my best to ‘do the experiment’ ....

Most of my cellar has a floor of (in many cases cracked) unglazed quarry tiles, about 1 cm thick, laying directly on soil and, at this time of year, they are decidedly ‘very wet’. I have just tried putting a roughly 9” square of copper sheet on the wettest part, weighed down (for ‘good contact’) with a sledge hammer.

By various nefarious means (details of which I will not describe here!) I have estimated that the resistance of the path to earth through this plate is about 10.4 kΩ (when measured at ~240V AC). If one adds on a notional 1 kΩ for body resistance, this would equate to a current of about 20.2 mA through a person who was standing on this floor with bare feet and touching a 230V source.

That’s certainly more than high enough a current to be worried about, but it has to be realised that this figure relates to a situation which is probably far ‘worse’ than one would ever be likely to encounter in almost any real-world bathroom. In the presence of even the slightest apology for 'floor covering' (and/or any 'anti-damp' measures under the tiles), it is likely that the current would fall to an extremely low, quite probably 'negligible', figure.

Kind Regards, John

 

[John D v2.0]

So you made an extraneous conductive part by laying a copper plate on the floor! Better get that bonded, especially if you're on tn-c-s!
Although not sure what happens if your foot needs bonding, if you're standing in wet mud!

 

[ericmark]

If I am reading this right?
What we are talking about is the amount of current that can flow into a floor rather than any metallic item?
I remember as an apprentice asking the question is earthing items actually making them safer or more dangerous. With an IT supply as with a bathroom shaver point it is hard to get a shock, however since then I have worked on a tunnel boring machine which had a delta 220 volt supply so IT and experienced first hand the problems. It was quite shocking.
Although at first glance

if you're standing in wet mud!

seems daft as you have stated you are looking already at something worse than normal, it does raise the question what is the worse and normal case. Skin is not rigid it will form to the surface, so a copper plate does not emulate skin, the contact also needs to mould to the surface. Bag of damp charcoal or other conducting material which will form to the surface.

 

[JohnW2]

So you made an extraneous conductive part by laying a copper plate on the floor! Better get that bonded, especially if you're on tn-c-s!

The copper plate was merely a convenient way of connecting, and would be broadly analogous to an internal metal pipe which was connected (by plumbing) to an incoming extraneous-c-p. In my case it was the floor, not the copper plate, which was the "extraneous-c-p" which one could argue should be bonded!

At least my installation is TT, but that does not remove the theoretical requirement to bond the cellar floor - in the case of an L-CPC fault in the installation, my MET and everything connected to it could rise to nearly L potential, but the cellar floor (mostly fairly distant from the earth electrode) would remain close to true earth potential.

Kind Regards, John

 

[JohnW2]

If I am reading this right? What we are talking about is the amount of current that can flow into a floor rather than any metallic item?

Exactly - that's what I was investigating.

It was in response to some exchanges about a highly hypothetical scenario in which some vandals had (in an external meter cabinet) disconnected the installation's earthing conductor from the incoming earth and, instead, connected it to the incoming L. I was arguing that, if all required CPCs and bonding were satisfactorily in place, that act should not create a significant shock risk since the whole place still ought to be roughly an equipotential zone, with everything 'touchable' being at roughly L potential.

However, it was then suggested that, in that situation, someone standing with bare feet on a wet concrete bathroom floor would be at risk if they touched some exposed metal (at ~L potential), such as a pipe or tap. In turn, I said that I thought it would be extremely unusual for a bathroom floor to have a low enough impedance to true earth for the resulting current to be, in itself, potentially life-threatening. I therefore did the experiment, which I think has more-or-less confirmed what I suspected, given that my test situation was far 'worse' than virtual all real-world bathroom floors.

Kind Regards, John

 

[John D v2.0]

I guess this is why lawn mowers, hedge trimmers, etc are mostly class II equipment, in case you're kneeling in a wet pond while holding them!

 

[JohnW2]

I guess this is why lawn mowers, hedge trimmers, etc are mostly class II equipment, in case you're kneeling in a wet pond while holding them!

Indeed. In the presence of faults (supply-side or within the installation) 'exporting' an installation's 'earth' outside of the equipotential zone (and particularly if into a place where 'true earth' exists) can be hazardous - whether it is 'exported' to the exposed-c-ps of an item of Class I equipment, or anything else. An outside tap plumbed in metal which is in electrical continuity with bonded pipes is a classic example.

Particularly with TN-C-S or TT (but TN-S is not immune), a fault can theoretically result in the potential of the installation's 'earth' (hence the potential of the 'equipotential zone') rising to considerably above true earth level - in the worst cases close to line potential,

Kind Regards, John

 

[ericmark]

I see so we are looking at the gradient so a house with TN-C-S where there is a fault causing the bonding to be at line potential needs to be a reasonable distance from another building with a TT supply for both to be safe.

This is where I argue to have a caravan parked next to a TN-C-S house supplied with a TT supply is dangerous where the meter cabinets for the house are only an arms length from caravan. However on a caravan site where fire regulations requires 15 foot between building and caravan the danger is much reduced.

The only large IT installation I have worked on was a tunnel boring machine under Hong Kong with an iron laden water dripping throughout the tunnel, the system seemed to swap every day as to which of the three phases would be at earth potential, I think every one had neon screwdrivers so you could test any metal before touching, we were getting shocks all the time. However no one was injured with the shocks. It was unpleasant, as was the water dripping on you all day every day. But no one died.

It did show me why we don't have IT supplies, there are not only shocking, but a real pain to trace faults with, although we shouldn't I am sure we have all used earth as a reference point, however that does not work with IT supplies.

In fact even touching live wires, in most cases it's not fatal, that's not to say it does no harm, 28 in 2010 not very high, so all in all one would need to be very unlucky to be killed due to the line and earth being swapped, and to get the intended person would be hard to ensure, so all in all I am sure there are better methods to murder some one.

 

[JohnW2]

I see so we are looking at the gradient so a house with TN-C-S where there is a fault causing the bonding to be at line potential needs to be a reasonable distance from another building with a TT supply for both to be safe.

I don't think that has got much to do with what we were discussing, or the experiment I did. Nor am I convinced that the (rare) hypothetical situation you describe would necessarily be a problem if all the required bonding was satisfactorily installed in the building with a TT supply, since the interior of that building should remain an equipotential zone.

This is where I argue to have a caravan parked next to a TN-C-S house supplied with a TT supply is dangerous where the meter cabinets for the house are only an arms length from caravan.

As I just wrote, 'exporting' an installation's 'earth' into an environment where there are potential paths to true earth (i.e. outside of the building's equipotential zone) is always potentially hazardous - and that remains true whether the supply is TN-C-S or TT (less likely, but not impossible, with TN-S).

A metal-plumbed outside tap within touching distance of, say, a metal soil pipe (which enters the ground) is, I think, as potentially dangerous as the caravan situation you describe.

Kind Regards, John

 

[winston1]

Recently in a hotel in foreign parts I was standing on a wet concrete bathroom floor and touched the shaving mirror to turn on its light. I got a belt but lived to tell the tale. I immediately called hotel reception who called maintenance man. He checked it (with a neon screwdriver), then proceeded to remove it from the wall using the same screwdriver. Found the earth was disconnected and the line was touching the metalwork. He disconnected everything and tucked the wires inside the metal back box with some tape. All this was done live.

So a wet bathroom floor was not a good enough earth to kill in this occasion. Glad though I was not touching a tap at this time.

I know someone will ask so I'll tell you, it was Dubal.

 

[JohnW2]

... Found the earth was disconnected and the line was touching the metalwork.

That was an unfortunate (and I imagine incredibly rare) combination of two 'co-incidental' faults!

So a wet bathroom floor was not a good enough earth to kill in this occasion. Glad though I was not touching a tap at this time. ... I know someone will ask so I'll tell you, it was Dubal.

I'm pleased to hear that it was not a good enough earth to kill, but I doubt thatt you would have got much, if any, of a shock in the typical UK domestic bathroom I was talking about ... I suspect that the hotel you're talking about had concrete floors absolutely full of earthed metal. Edit: ... or maybe with UFH from which you were being 'protected' by a well-intentioned 'earthed screen'.

Kind Regards, John

 

[ban-all-sheds]

Indeed. In the presence of faults (supply-side or within the installation) 'exporting' an installation's 'earth' outside of the equipotential zone (and particularly if into a place where 'true earth' exists) can be hazardous - whether it is 'exported' to the exposed-c-ps of an item of Class I equipment, or anything else.

I still like to have a cpc in the flex. It may not connect to anything in the appliance, but it greatly increases the chances of damage tripping the RCD via L or N ->E contact via the cpc. Call me old fashioned, but I'd prefer that than wait until there's L->E contact via me.

 

[JohnW2]

I still like to have a cpc in the flex. It may not connect to anything in the appliance, but it greatly increases the chances of damage tripping the RCD via L or N ->E contact via the cpc. Call me old fashioned, but I'd prefer that than wait until there's L->E contact via me.

We've discussed this before and, as you will probably recall, I totally agree with you. I'm actually rather surprised that a CPC is not 'required'.

Kind Regards, John