Shocking

[sidd]

I don't know if anyone here can help with an electrical problem.

The plumber had taken off a radiator upstairs, and allowed some water to go through the floorboards, where it trickled into a smoke alarm. He was given a strong shock when he lifted the floorboards to remove the radiator pipes. Then he left with the other smoke alarms going off and not mentioning any of this to anyone. When my wife got back she called me in a bit of a panic, saying the kitchen lights were out as well.

After denial from the plumber that he had let any water through the floor, he finally agreed there could be no other explanation for water in the smoke alarm.

Apart from an incompetent plumber what worries me is that the electrics have supposedly been (95% finished) done to 17th edition standards. As such, I am wondering why no RCD kicked in.

For a 6000ft2 house, there are two consumer units, with only three frontline RCDs, one of which covers a mixture of rings and lights and the other 2 cover lights (inside and out) and rings (outlets of all types) respectively. Type B circuit breakers have been used in a MK Sentry box.

I guess that if MK make a consumer unit that is 17th compliant using this sort of configuation then it must be legal, but it is demonstrably ineffective for protection.

Does anyone know enough about this to help me with any ammunition when I confront the sparkies?

 

[Schoolmarm]

So what was wrong with the kitchen lights? Had the RCD not knocked them off?

 

[Flyfisher]

Do you only have the plumber's word for his "strong shock" ?

My understanding is that RCDs are intended to reduce fatal electric shocks (by limiting current and time below a 'safe' curve). They don't eliminate all possibility of a shock, and a person's judgement as to the severity is likely to be very subjective. Clearly, if the plumber did receive a shock, it wasn't fatal and, as Schoolmarm noted, the kitchen lights were out so does that imply that the RCD did operate? A mild shock (possibly exaggerated by the plumber) followed by the RCD tripping sounds like a reasonable scenario to me. Having said that, and given the "95%" nature of your wiring, it's still a good idea to talk about the incident with the electricians.

Also, if you're messing around with wiring and manage to insert yourself between the live and neutral conductors then you'll receive a shock (quite possibly fatal) and the RCD, quite correctly, will not trip. Don't become complacent just because RCDs are fitted.

 

[sidd]

I don't really know, as my wife did most of the investigations and she is not very technical. But I assume that at some point the RCD tripped ALL the lights but she didn't notice that the rest were off. We have one slightly blackened smoke alarm and another that has been "permanently deactivated" by the sparkies when they came.

So assuming it did trip, that was after not one but three "events": water hitting wires to make a circuit with the woodwork, plumber touching water/woodwork and water damaging the smoke alarm.

 

[sidd]

Thats right, Flyfisher, the implication when he said he got a strong shock was that there was some loose wiring below the floorboards and he was the victim. And I think you are right that if neutral is involved the RCD won't trip, but I don't think neutral was involved here.

 

[Flyfisher]

These things are difficult to really pin down when there is little, or secondhand, information. If 'loose wiring' means bare wiring then that must surely be an installation issue. But if the shock was caused by water getting into a junction box that the plumber subsequently touched while fumbling around under floorboards, then that sounds like an accident and one where the RCD did its job correctly by tripping.

Because an RCD cannot trip until it has detected a current imbalance (i.e. current leaking out of the circuit, through the plumber in this case!) it's quite possible that the shock was felt just before the RCD tripped. I don't think this necessarily implies a faulty RCD. The plumber might also be looking for any excuse to divert attention from himself causing the water leak in the first place.

As for the three events; water between wires and dry woodwork might not cause much (electrical) problem as the dry wood wouldn't be a good conductor, water in the smoke alarm could cause damage to the alarm circuitry without necessarily causing a current 'leak', but the plumber would be a better conductor and could have been the decisive event in terms of the RCD tripping, but not before he had received a shock - mild in the physiological sense because of the RCD protection but severe in the psychological sense because it was completely unexpected and, frankly, probably quite frightening.

 

[Feltwell]

You will get a shock even with an RCD fitted, if you're the one acting as the route to earth. I have had a shock before, it definitely hurt, the RCD did its job and tripped. No doubt it would have hurt more with no RCD.

30 milli-seconds is the usual trip time for an RCD, my firm sells the plug-in type for garden / power tool use. You can slice through a cable whilst trimming a hedge for example and not feel a thing - so long as you are not acting as the earth conductor to make the RCD trip. 30 milli-seconds is plenty of time for it to be felt as a sharp burst of pain if you are the conductor, but if you're fit & healthy it's unlikely to prove fatal.

 

[chuckey]

There are at least two sorts of RCDs in common use. A 100mA one, this is for plant protection which is on the incomer side of the consumer unit, so every thing goes through this. Then there could be a 30 mA one on one or more specific circuits for personnel protection. You some times can get away with a 30 mA RCD for the whole house, but I have found to my cost that they can be subject to random tripping.
So the question is what value was the RCD and which circuits did it protect? It would be barmy to have a 30 mA RCD just protecting some kitchen lights. And yes you can get a very small shock with a 30 mA RCD, but only about the same as static from combing your hair, noticeable but not painful.
Frank

 

[sidd]

Thank for the thoughts. I think I need to get it checked out properly.

 

[Flyfisher]

You will get a shock even with an RCD fitted, if you're the one acting as the route to earth. I have had a shock before, it definitely hurt, the RCD did its job and tripped. No doubt it would have hurt more with no RCD . . . . . 30 milli-seconds is plenty of time for it to be felt as a sharp burst of pain if you are the conductor, but if you're fit & healthy it's unlikely to prove fatal.

I recall once seeing a graph of current vs time showing various curves of seriousness; e.g. 30mA for an extended period of time could be fatal whereas it would be "safe" for a few tens of milliseconds. I can't remember the RCD trip time but the idea is to keep the leakage current vs time well below the fatal curve. But "safe" is a statistical thing; as Feltwell rightly says if you're fit and healthy you'll most likely be unharmed but it can't be totally guaranteed, so don't take unnecessary risks just because you have RCDs fitted!

The route of the current through the body also makes a big difference and hand-to-hand shocks (i.e. across the chest) are among the most dangerous. For this reason, it's common practice for anyone working with live circuitry to keep one hand behind their back or in their pocket. Another good practice is never to work alone.

Of course, the safest practice of all is to leave well alone!

 

[Lime]

It is a common misconception that RCDs reduce current.
They don't.
They only minimise the time the current is passed.

 

[outlaw]

I havent read all the thread as I am passing through this morning, silly it may sound but, does the shock get worse with a better connection, like wet hands?
I dont want to experiment so thought I would ask instead

Tony

 

[Lime]

I havent read all the thread as I am passing through this morning, silly it may sound but, does the shock get worse with a better connection, like wet hands?
Tony

Yes. :shock:

 

[Flyfisher]

Yes. A shock is the result of electrical current flowing through the body. The lower the resistance path to ground, the more current will flow, the bigger the shock will be. So a wet hand touching a live wire while leaning on an earthed copper water pipe with the other hand, or standing barefoot in a puddle, will be very nasty indeed. The same, but dry, hand touching a live wire while standing on a fibreglass ladder in wellington boots and with your other hand in your pocket will be somewhat less traumatic.

If an RCD trips the circuit it's possible that the RCD limits the current/time to the same intensity even under different shock conditions, but I'm only guessing at this as it's not the sort of thing I spend time experimenting with. :wink:

Incidentally, if there is no path to ground at all then no current can flow and there will be no shock. This is why birds can stand on high-voltage power lines with no ill effects and it's the same principle that allows people to work on live 500,000 volt power lines.
Some amazing film of this here: http://www.youtube.com/watch?v=9tzga6qAaBA&NR=1

 

[MdB]

Incidentally, if there is no path to ground at all then no current can flow and there will be no shock. This is why birds can stand on high-voltage power lines with no ill effects and it's the same principle that allows people to work on live 500,000 volt power lines.
Some amazing film of this here: http://www.youtube.com/watch?v=9tzga6qAaBA&NR=1

You do not need a path to ground to get a shock. Grab the live and neutral or two phases of a three phase system and you will get a large shock (and the RCD won't trip).

Oh and if you really want to receive a good shock, try salt water rather than plain water....

 

[Flyfisher]

Fair point about 'potential difference' being the key rather than ground per se (although the neutral conductor is essentially a ground connection).

As for three-phase systems, I've always avoided them like the plague after reading that you'd be unlucky to be killed by a typical single-phase shock (240V) whereas you'd be lucky to survive a typical three-phase shock (415V). I'm living testament to the former but have no intention of risking the latter.