Supplementary bonding sounds dangerous!

I know that supplementary bonding is supposed to be safe, I get the Faraday Cage analogy, and I've read the DIYNOT wiki but I'm still struggling with the principle. According to the Electrical Safety Council, the definition is as follows:

"Supplementary bonding
The connecting together of the metal parts of electrical equipment (such as a heated towel rail) and the metal parts of a non-electrical item (such as pipes) to prevent a dangerous voltage between them, if a fault occurs. May be required in bath and shower rooms."

So if a fault does occur, the heated towel rail becomes live. Since it's connected to the bath taps through the supplementary bonding, presumably the taps are also live. So you could be sitting in water, in the bath, touching a live tap...

Doesn't sound particularly safe to me.
Can anyone explain why it is?

I'm not an expert on this, but it would have to be a serious fault to make everything live - if a fault occurred in one piece of equipment, any fault current should flow through the CPC of something else due to the supplementary bonding. Also, even if everything were to go live, the point is that you only get a shock due to a potential difference, i.e. being in contact with two things at a different potential, if everything is at the same potential (be it earth or 240V) you can't get a shock...

..

ok, so a scenario for you ( and a recent real one at that.. )

through some fault, the towel rail casework ends up live and doesn't blow the breaker.. ( screw through a cable severing the upstream cpc and connecting the downstream to live say.. )

if your heater isn't bonded to everything else, and you happen to be running a bath and brush your bum against the still live heater then you've got 240V against your bum, and your hands on a tap that will be at, or close to earth ( through the pipes back to the main bond at the stop cock )

if your heater is bonded to your taps and everything else then there's still a path to earth via the pipes and the main earth at the stop cock and it should trip the breaker..

now another scenario, there is a fault on the boiler that puts 60V or so onto the heating pipes, if the radiator in the bathroom isn't bonded to the taps etc then same as above, you brush against the radiator while turning on a tap and then you've got 60V or so accross you..

An electric shock is when current flows through a part of the body.

That requires two parts of the body to be in contact with two items that are at different potentials. Contact with only one item no matter if it be Live at 230 volts will NOT provide a shock. Birds can roost on wires at 11,000 volts and not receive a shock. But if two birds on different wires touch each other then they will be both shocked as they complete a circuit between wires at different potentials. Two birds on the same wire can touch each other as there is no potential difference between them.

In the home if all exposed metal work is bonded together then touching any two metal items cannot result in a shock as all metal is bonded and therfore at the some potential. That potential could be 230 volts and still be safe.

The problem however is how far does one extend the area. In theory maybe each room can be treated as an area on the basis that it would be difficult to touch metal in two different rooms at the same time.

But if a fault means all bonded metal parts are at 230 volts in one room and someone does manage to touch metal in that room and metal in another room that is earthed then a shock will be the result.

Once more theory is applied and any fault that leads to metal work being at 230v should be detected by either excessive current or earth leakage but that assumes the metal that is bonded together is also connected to the protective earth of the electrical system. If that connection is not there or has failed then it is possible that there will be no excessive current flow to trip the MCB and no un-balance to trip the RCD.

If items are not bonded and a fault makes some metal work live while other metal in the same room is earthed then any one touching the live metal and the earthed metal will get a shock. If it is a prolonged shock then the RCD if fitted MAY cut the power, if the shock current through the body is less than the RCD's trip current the RCD will not trip. That current may still be enough to be fatal. Or if the RCD does trip the effect of the short shock before the trip occurs may be to cause the person to fall, drops and spill a container of boiling water or in some other way suffer an injury.

Me. ? I would bond all exposed metal work and ensure it was connected to the earth.

There needs to be a difference in potential between the two points of contact in order for current to flow.

Another scenario: (I'm not suggesting that you try this, of course!): You could walk barefoot along the conductor rail on an electric railway and not feel a thing, so long as you keep both feet on that rail and don't touch anything else. Your whole body will rise to a potential of several hundred volts, but because both feet are at the same voltage on that live rail, there is no difference in potential between your feet and thus no current flows. As soon as you tried to put one foot on the ground or a running rail, or grabbed hold of some trackside accessory with a hand, however, a potential difference exists across two points of your body, current flows, and you get zapped.

It's the same principle as birds being able to sit on power lines - Both feet on the same line and therefore at the same potential, so no current flow through the bird, even though its whole body will be at hundreds or thousands of volts.

Edit: Cross-posted with Bernard.

There are companies who "land" men onto 132,000 volt over head lines to carry out repairs while the line is live

http://www.chopperworx.co.za/powerline.html

In these cases there are dangers of corona discharge and the AC current that flows from the wire into the, at this voltage, significant capacitance of the man, helicopter and equipment being used.

see http://www.youtube.com/watch?v=oDW3iBYya3I

bernardgreen said:

Me. ? I would bond all exposed metal work and ensure it was connected to the earth.

Click to expand...

What do you mean by "exposed"?

Thanks for all those very informative replies. So, from what everyone is saying, it seems it is 100% safe to sit in a bath full of water and touch a tap that is live?

That isn't what everyone is saying.

It might be 100% safe.

It might be 100% dangerous.

It depends.

RegB said:

Thanks for all those very informative replies. So, from what everyone is saying, it seems it is 100% safe to sit in a bath full of water and touch a tap that is live?

Click to expand...

Assuming the bath and its drainage, if metal, is properly supplementary bonded to the tap pipes (and anything else you might be poking whilst in this bath that should be bonded), then yes.

In that scenario, how could the tap be "live"?

RegB said:

Thanks for all those very informative replies. So, from what everyone is saying, it seems it is 100% safe to sit in a bath full of water and touch a tap that is live?

Click to expand...

Assuming you have a plastic waste pipe and plastic bath, why do you feel that water would make this any more dangerous than sitting in an empty bath?

ban-all-sheds said:

In that scenario, how could the tap be "live"?

Click to expand...

All pipes and exposed metal surfaces are bonded to each other.

One of the bonded surfaces is the case of an appliance the cable to which is damaged.

The damage has severed the earth wire in the cable and exposed the copper of the live wire.

The severed end of the earth wire, the end that is connected to the appliance, has come into contact with the exposed Live conductor.

If there are no other earthed items among the bonded items then those bonded items are all live.

Paul_C said:

Another scenario: (I'm not suggesting that you try this, of course!): You could walk barefoot along the conductor rail on an electric railway and not feel a thing, so long as you keep both feet on that rail and don't touch anything else. Your whole body will rise to a potential of several hundred volts, but because both feet are at the same voltage on that live rail, there is no difference in potential between your feet and thus no current flows. As soon as you tried to put one foot on the ground or a running rail, or grabbed hold of some trackside accessory with a hand, however, a potential difference exists across two points of your body, current flows, and you get zapped.

Click to expand...

Remember also to only do this while walking facing the direction of traffic, as your demonstration will be somewhat spoiled if you are run over by a train.