Main Bonding and Cross Bonding question?

[Megavolt]

Looking at pic that is a LPG bulk tank supply or a gas supply from a exsternal gas meter. I don't think you will find a meter in that house.

 

[Knobhead]

By GOD Sir, I think your right!

Buggered up my idea for free gas!

 

[chrisgc]

Looking at pic that is a LPG bulk tank supply or a gas supply from a exsternal gas meter. I don't think you will find a meter in that house.

Get the trumpets out - you are indeed right - the gas tank is around an outbuilding - I looked this morning.

So where does the gas main bond go then??

 

[telectrix]

if it's coming from the tank in plastic, the question is does it need bonding?

 

[ackbarthestar]

1/ It appears that you have a 16mm2 earthing conductor from the electrode
2/ Main protective bonding should be applied to all metal services entering the property although it might be argued from the pictures that the gas pipe may not be extraneous.
3/ A long lead test to ensure there is sufficient continuity between all the metalwork
BTW the 415.2.2 does not necessarily mean a value of 0.05 Ohms must be achieved since it relates to Ia or the possible fault current that might flow from the largest appliance/circuit in the vicinity of the pipework.

R=50/Ia. If the only electrical circuit is supplying a 3kW heater element then the Ia will be, worst case, 5 times the rating of the MCB. In this case is likely to be 16A times 5 = 80A ( most likely 50A) which will require an overall resistance of between 4.6 - 2.8 Ohms. However, if the simultaneously accessible metalwork is next to a 10.5kW instantaneous water heater then the likely fault current will be in the order of 0.9 Ohms. These are the worst cases which is why a 0.05 Ohms value is used to ensure that there is a minimum measurable resistance value between pipework .

I hope that provides sufficient smoke to clear the issue:dizzy2::cheesy:

 

[ezzzekiel]

cross bonding was and is part of the gas regs (but for some obscure reason they seem to think its our responsability)

0.05 ohms reading on main bonds is the same rule as supp bonds, it ensures that under single fault conditions a potential difference of -+ 50v (safe touch) does not appear on the pipework, by limiting the resistance (for 10mm) to 0.05ohms upto 27metres(or there abouts)

alternatively you could calculate the actual size using Rcalc allready provided.

 

[ackbarthestar]

R=50/Ia. If the only electrical circuit is supplying a 3kW heater element then the Ia will be, worst case, 5 times the rating of the MCB. In this case is likely to be 16A times 5 = 80A ( most likely 50A) which will require an overall resistance of between 4.6 - 2.8 Ohms. However, if the simultaneously accessible metalwork is next to a 10.5kW instantaneous water heater then the likely fault current will be in the order of 0.9 Ohms. These are the worst cases which is why a 0.05 Ohms value is used to ensure that there is a minimum measurable resistance value between pipework .

This maybe true for disconnection times but doesn't explain, or doesn't prove that the voltage on the pipework will be below 50V.

Now apply the fault current to the maximum touch voltage allowed and we get a maximum resistance of 50/80 = 0.625 Ohms and for a larger load of 10.5kW we have 50/250 = 0.2 Ohms.

I would have thought somebody would have noticed that ! It must be the weekend when all go to sleep

 

[ezzzekiel]

doesn't explain, or doesn't prove that the voltage on the pipework will be below 50V.

its not meant to its to ensure there isnt a difference of 50v between any pipepwork.



Protective bonding, purpose to ensure in the event of a fault all exposed/extraneous conductive parts will be live at same potential, in order to achieve this it is recomended that the resitance of the conductor does not exceed 0.05ohms.

 

[ackbarthestar]

0.05 Ohms is what we might call a rule of thumb. It doesn't have to be that value as long as the resistance between simultaneous metal parts is no larger than 50/Ia (type B MCB) for the largest local load. Which for a lighting circuit might be 1.67 Ohms and for a 10.5kW instantaneous water heater 0.2 Ohms.

 

[ezzzekiel]

to achieve this it is recomended that the resitance of the conductor does not exceed 0.05ohms

apart from that the formula relates to the sizing of supplementary bonding conductors where disconnections times can not be met we are not talking about supp bonds but main bonds which is to ensure no difference in potential will occur between parts.

 

[ackbarthestar]

to achieve this it is recomended that the resitance of the conductor does not exceed 0.05ohms

Try BS7671 415.2.2 otherwise it would have stated it.

 

[ackbarthestar]

to achieve this it is recomended that the resitance of the conductor does not exceed 0.05ohms

apart from that the formula relates to the sizing of supplementary bonding conductors where disconnections times can not be met we are not talking about supp bonds but main bonds which is to ensure no difference in potential will occur between parts.

Lol, I suppose we are not talking about cross bonding then? Where did the disconnection times not being met come from?
I thought we are discussing the effectiveness of supplementary (cross) bonding to clamp the voltage below the maximum touch voltage even when disconnection times are met and where a fault might occur.

However, It is unlikely that cross bonding would be required for normal locations that are not within a bathroom. As long as the main protective bonding is in place and the disconnection times were met which is likely to be a lot less than 0.4s (0.01s).
The rule of thumb for the main protective bonding is as you say is 0.05 Ohms....

 

[ezzzekiel]

yup crossed wires (prob my fault for not reading all of thread)

 

[palmerelectrcal]

gas safe eng wouldnt connect gas meter until cross bonding in place from gas to boiler pipes.!!!!!!