Earthing a steel bath if fusebox has an RCD?

[bernardgreen]

When I bought this cottage the cast iron bath had a steel waste pipe that went to the outside through a damp (and therefore conductive) wall. About 6 inches from the pipes's hole and fixed to the wall was the hopper and metal down pipe that went into the ground. A path ( metal, damp stone / mortar, metal ) existed between the cast iron bath and ground. I didn't measure the impedance. To me that was a low impedance path making the bath a candidate for bonding.

( That bath has gone and the steel pipe that was holding up some of the floor boards has also gone. )

 

[ban-all-sheds]

Are you suggesting that everything with an electrical connection ('good', 'bad' or 'indifferent') to a (main bonded) extraneous-c-p thereby becomes an extraneous-c-p itself?

Are you suggesting that it is not?

Consider a coper pipe with a compression (i.e. a screwed) coupler joining it to another copper pipe which enters the bathroom. Are you suggesting that the second pipe is an e-c-p, and therefore a candidate for supplementary equipotential bonding, but a metal bath connected by a screwed connection to the first pipe is not?

 

[JohnW2]

Consider a coper pipe with a compression (i.e. a screwed) coupler joining it to another copper pipe which enters the bathroom. Are you suggesting that the second pipe is an e-c-p, and therefore a candidate for supplementary equipotential bonding, but a metal bath connected by a screwed connection to the first pipe is not?

No. I'm suggesting that anything (whether pipe. bath door handle or whatever) is only an extraneous-c-p if it is "liable to introduce a potential" (other than the potential of the MET/bonded pipework etc.), and that if it is not an extraneous-c-p that the question of supplementary bonding does not arise (whether it is a pipe, bath, or anything else).

Kind Regards, John.

 

[RF Lighting]

When I bought this cottage the cast iron bath had a steel waste pipe that went to the outside through a damp (and therefore conductive) wall. About 6 inches from the pipes's hole and fixed to the wall was the hopper and metal down pipe that went into the ground. A path ( metal, damp stone / mortar, metal ) existed between the cast iron bath and ground. I didn't measure the impedance. To me that was a low impedance path making the bath a candidate for bonding.

( That bath has gone and the steel pipe that was holding up some of the floor boards has also gone. )

If the pipe was introducing a potential into the house then it should have been main bonded just like your water and gas / oil supply. This has been the case for many years and remains the same today.

 

[JohnW2]

When I bought this cottage the cast iron bath had a steel waste pipe that went to the outside through a damp (and therefore conductive) wall. About 6 inches from the pipes's hole and fixed to the wall was the hopper and metal down pipe that went into the ground. A path ( metal, damp stone / mortar, metal ) existed between the cast iron bath and ground. I didn't measure the impedance.

As I said, that is one of the unusual situations in which (as it did for you) inspection/observation will reveal that the waste pipe was potentially an extraneous-conductive-part.

To me that was a low impedance path making the bath a candidate for bonding.

Yes, but I think two things are getting confused here - since we're now into something which goes way beyond supplementary bonding. If, as you say, you feel that the waste pipe needs to be considered as an extraneous-c-p ("liable to introduce true earth potential" into the building), the it would require MAIN bonding, not just (possibly) supplementary bonding.

Edit: typed too slowly again!

Kind Regards, John

 

[ban-all-sheds]

Consider a coper pipe with a compression (i.e. a screwed) coupler joining it to another copper pipe which enters the bathroom. Are you suggesting that the second pipe is an e-c-p, and therefore a candidate for supplementary equipotential bonding, but a metal bath connected by a screwed connection to the first pipe is not?

No.

We need to stop your answer there.

What other things you are suggesting are irrelevant.

You are not suggesting that the bath is not an e-c-p.

 

[bernardgreen]

If the pipe was introducing a potential into the house then it should have been main bonded just like your water and gas / oil supply. This has been the case for many years and remains the same today.

What about other metal items in old buildings ( or even new buildings ) with walls that are conductive to some degree ? Steel tie bars that are preventing walls from bowing out. They are not service supply items but they can introduce Ground potential into the equipotential zone. Admittedly it is not going to be a very low impedance but it could certainly be low enough allow some current to flow if a live conductor ( or person in contact with Live ) .touches it.

The question is :- Would the impedance be low enough to create a significant shock to a person while being high enough to limit the current to less than the trip current of an RCD on the circuit from which the Live is coming ? if the answer is yes then I would say bond or earth it to ensure the RCD could trip if needed.

 

[JohnW2]

You are not suggesting that the bath is not an e-c-p.

I am suggesting that something which is not an extraneous-c-p is not an extraneous-c-p. If it is entirely contained within a 'location', it cannot possibly "introduce (or be liable to introduce) a potential" (any potential) into that location (any more than can a door handle or spoon).

If something is liable to introduce a potential into the location, it must be something other than the ('entirely within the location') bath, and should be main-bonded, anyway.

Kind Regards, John

 

[JohnW2]

The question is :- Would the impedance be low enough to create a significant shock to a person while being high enough to limit the current to less than the trip current of an RCD on the circuit from which the Live is coming ? if the answer is yes then I would say bond or earth it to ensure the RCD could trip if needed.

As both RF and I have said, if you believed that the item was potentially an extraneous-c-p it would be require main bonding - this would not be a job for a 2.5mm² (protected) or 4mm² bonding conductor!

Kind Regards, John

 

[Spark123]

Without the idea of a numpty dropping their hair dryer in the bath (which is their own fault), I fail to see any reason why supplementary bonding a metal bath* when the pipework to it is already effectively bonded make one iota of a difference.
(*Where the only reason for the bath possibly being an ecp is the presence of metal pipes)

 

[bernardgreen]

If the extraneous voltage is from a low impedance source ( from a metallic water main for example ) then the bond may have to carry a high current when the MET goes a few volts above ground. So main bond with 4 or 10 mm² cable. But the source impedance of the extraneous voltage is high then the current is unlikely to be more than a few amps so the bond wire could be 1 mm² cable.

When a 30 mA Δ RCD is fitted the maximum that can flow into any extraneous metal from the Live or Neutral conductors is about 30 mA more than that and the RCD will trip.

So why is the main bond specified as being at least 4 mm² and preferably 10 mm² ? Could it be to try and cope with the current that can flow from the MET to the extraneous conductor, current that does not pass through any protective device.

 

[RF Lighting]

If the pipe was introducing a potential into the house then it should have been main bonded just like your water and gas / oil supply. This has been the case for many years and remains the same today.

What about other metal items in old buildings ( or even new buildings ) with walls that are conductive to some degree ? Steel tie bars that are preventing walls from bowing out. They are not service supply items but they can introduce Ground potential into the equipotential zone. Admittedly it is not going to be a very low impedance but it could certainly be low enough allow some current to flow if a live conductor ( or person in contact with Live ) .touches it.

The question is :- Would the impedance be low enough to create a significant shock to a person while being high enough to limit the current to less than the trip current of an RCD on the circuit from which the Live is coming ? if the answer is yes then I would say bond or earth it to ensure the RCD could trip if needed.

Bond bond bond bond bond bond bond bond bond bond bond bond bond bond bond bond bond bond NOT earthing.

Don't care if it happens to earth the extraneous part or not. That's not what we are trying to achieve, and we are certainly not trying to cause RCDs to trip.

We are simply aiming to create a faraday cage by bonding everything together. Until you can grasp this most basic of principals, it's not surprising you get so confused and have such a mis-trust of earthing systems.

 

[ban-all-sheds]

I am suggesting that something which is not an extraneous-c-p is not an extraneous-c-p. If it is entirely contained within a 'location', it cannot possibly "introduce (or be liable to introduce) a potential" (any potential) into that location (any more than can a door handle or spoon).

It's a metal bath.

It is in the room.

Unlike the door handle and the spoon it is electrically connected to something which can be at a different potential to exposed conductive parts in the room.

Why is it not an extraneous-conductive-part?

 

[JohnW2]

(*Where the only reason for the bath possibly being an ecp is the presence of metal pipes)

You seem to be talking like BAS. If the metal pipes are extraneous-c-ps then they are extraneous-c-ps (which require main bonding). However, something (like a bath) which happened to be in electrical continuity with that extraneous-c-p would not automatically (in fact, not normally) itself become an extraneous-c-p. If it did, virtually every pipe and every protective conductor within a building would qualify as an extraneous-c-p - which not only would be silly, but would indicate a lack of understanding of what the concepts of extraneous-c-ps and main equipotential bonding are all about!

Kind Regards, John

 

[bernardgreen]

DO NOT TRY THIS

Consider a plastic bath of water with no metal drain pipe. Sitting in such a bath when some one drops a hair dryer into the water is not always fatal, in fact the shock may be little more than a severe tingle. The voltage gradient from Live to Neutral is concentrated in the water in and closely around the hair dryer. The voltage gradient in the rest of the water is unlikely to be steep enough to create a severe shock.

But the water will be at a potential mid way between Live and Neutral ( approx 120 volts ) . The bath is a mini equipotential zone. And the RCU will not trip, there is no earth leakage.

The danger is when the person is climbing out of the bath and touches something that is earthed or bonded. The leg that is still in the water is at 120 volts and the hand touching the heated towel rail is at "earth" ( Neutral ) potential. OUCH or fatal.

DO NOT TRY THIS as there are arrangements of body and hair dryer where the voltage gradient around the hair dryer will also be close enough to the body to create a serious and possibly fatal shock.