Why should temporary continuity bonding clamps be used if cutting in to a copper water/central heating or gas pipe. Is it something to do with if the pipe is live you would become the pipes path to earth?
Temporary continuity bonding clamps
- Thread starter Fredstudio
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With gas,it prevents the possibility of spark and subsequent ignition of gas left in pipe.
it also maintains an equipotential zone in the working area.
it also maintains an equipotential zone in the working area.
No.
There is that possibility, and in the case of a gas pipe, a risk of ignition.
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The risk of ignition when cutting in to a gas pipe is static from the pipe slice?
Where could the possibility of a spark come from?
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Gas pipe could be sparks, but for water can't see why once the water has entered the property why it should be bonded when using RCD protection.
If you touch a live wire and nothing bonded then there may be some capacitive or inductive linking which can cause a small shock, but the current would be very low, so in some ways safer if nothing bonded, (This is called an IT system and is what is used with shaver sockets) however in that case you may be unaware of live parts, and one could touch two live parts at a different potential, so we earth one live conductor so if the other one touches earth a fuse or other protective device will rupture/disconnect so making it safe before you touch it. However 13 amp is a lot of current, so we today use RCD's which will trip at between 15 and 30 mA. Normally around 25 mA.
The whole idea is they trip BEFORE you touch anything, they do take 40 mS to operate so you would still get a shock, but not for very long. Before the 30 mA RCD there is a possibility that a dog could knock over a standard lamp which could smash the bulb glass allowing the live filament to rest on a radiator, which may mean a radiator in another room also becomes live due to copper connecting pipes. Unlikely scenario I know, but still possible. However if the pipe is bonded, then it will likely blow a fuse. However even if not bonded, it would still likely trip a RCD.
The problem was plastic pipes and fittings, even with copper pipes, one can connect them with plastic push connectors, so near impossible to ensure all copper is bonded, I think it goes back to 14th edition when we went mad, with bonding metal window frames etc. And back in the 50's it was common to use the water supply as an earth, but again the use of plastic water mains messed that up.
So services be it gas, oil, water or other coming for outside the equipotential zone are bonded as they enter the zone, but after that point where RCD protection is used there is not real requirement.
Gas is the odd one out, earthing gas pipes with an earth from a TN-C-S supply must be after the electrical isolation block, so that with a loss of PEN high current is not transmitted by the gas pipe which could melt pipe and ignite gas. Water pipes are unlikely to melt, and if they do no real danger. Bonding as the pipe enters the property OK, but where the meter is outside, one needs to be careful not to get the bonding before the isolation block.
pre 2008 the bonding requirements were stricter, but there is a trade off.BS 7671:2008 said:
If you touch a live wire and nothing bonded then there may be some capacitive or inductive linking which can cause a small shock, but the current would be very low, so in some ways safer if nothing bonded, (This is called an IT system and is what is used with shaver sockets) however in that case you may be unaware of live parts, and one could touch two live parts at a different potential, so we earth one live conductor so if the other one touches earth a fuse or other protective device will rupture/disconnect so making it safe before you touch it. However 13 amp is a lot of current, so we today use RCD's which will trip at between 15 and 30 mA. Normally around 25 mA.
The whole idea is they trip BEFORE you touch anything, they do take 40 mS to operate so you would still get a shock, but not for very long. Before the 30 mA RCD there is a possibility that a dog could knock over a standard lamp which could smash the bulb glass allowing the live filament to rest on a radiator, which may mean a radiator in another room also becomes live due to copper connecting pipes. Unlikely scenario I know, but still possible. However if the pipe is bonded, then it will likely blow a fuse. However even if not bonded, it would still likely trip a RCD.
The problem was plastic pipes and fittings, even with copper pipes, one can connect them with plastic push connectors, so near impossible to ensure all copper is bonded, I think it goes back to 14th edition when we went mad, with bonding metal window frames etc. And back in the 50's it was common to use the water supply as an earth, but again the use of plastic water mains messed that up.
So services be it gas, oil, water or other coming for outside the equipotential zone are bonded as they enter the zone, but after that point where RCD protection is used there is not real requirement.
Gas is the odd one out, earthing gas pipes with an earth from a TN-C-S supply must be after the electrical isolation block, so that with a loss of PEN high current is not transmitted by the gas pipe which could melt pipe and ignite gas. Water pipes are unlikely to melt, and if they do no real danger. Bonding as the pipe enters the property OK, but where the meter is outside, one needs to be careful not to get the bonding before the isolation block.
Thank you. Where does the possibility of a spark come from when a gas pipe is cut?
Under a fault condition it could rise to the risk of electric shock/arcing due to possible differing potentials between them.
Couple of factors can influence arcing when disconnecting a gas pipe, even if it's redundant.
From a difference in potential, between the two pipe ends. Even the earth/ground/soil, even the air itself, can have a difference in potential - measured across two separated points.
Would this produce a small current but enough to make a spark?
Small, large, whatever. Even a tiny current can create a spark.
Metal pipes can carry electric current during a fault and also in normal operation.
Such current can be determined before cutting, but that's of no use because it can appear / change / disappear at any time.
If you cut that pipe while current is flowing in it, a spark will occur where the pipe is cut and if gas is leaking out, then it's explosion and fire time.
In some unlikely situations, a dangerous voltage can appear between the cut ends, anything up to 400+ volts.
That voltage won't be detectable until the cut is made, and it's then too late.
Connecting a temporary bond to the pipe at either side of the proposed cut before making the cut maintains electrical continuity and avoids both of these problems.
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