Why earth oil pipe at tank?

[stuban]

Hi chaps

Just quoted for a job based on a spec which included earthing a metal oil pipe to a new rod at the plastic tank about 15 yards from the house.

The house is out in the sticks but pme. The pipe is not plastic coated that I could see so is there any reason for this? I would think this is a no-no rather than being of any benefit.

I could maybe understand it if the pipe was insulated and the system TT(or is that rubbish) but assuming we bond where the pipe comes into the house is there any situation where you'd earth this pipe to a rod?

Thanks a lot
Stu..

 

[IQ Electrical]

Static from moving liquids?

 

[IQ Electrical]

If it is for static discharge, the pipe must still be bonded to the MET where it enters the building.

 

[johnboy6083]

Is it to reduce the touch potential of the tank and pipe, especially in earth fault conditions?

 

[malcolmsanford]

It's not very clear your post are you bonding the oil pipe coming into the house , which is a PME earth and then bonding the tank outside of the house with a rod.

If your bonding the pipe going into the house then that is bonding main services as you do with gas and water so pretty straight forward.

As for earthing the pipe at the tank all I can think is that they want to earth the system for when the tanker fills the tank the same as they do at petrol stations.

 

[netblindpaul]

As both above, though in this case I would push for a "proper" PME and want to drop a rod at the MET with a connecting conductor capable of taking full diverted N current and then a bit probably, I would not dream of less than a 16mm tbh.

 

[stuban]

I've not included the rod on our quote because I couldn't find a reason for it. The spec was from the previous quote and what the customer based their spec on.

Agree the bonding is a no brainer but being new to the business wanted to see if I was missing something with the rod at the tank.

Cheers
Stu..

 

[netblindpaul]

True PME mate, also as has been said static.
If the house earth (N) fails then any diverted N current will seek your rod at the tank 1st rather than look to earth through other means, you rod at the MET would also help in reducing diverted fault currents as would the tank rod.
I would say best practice rather than no reason.

 

[johnboy6083]

As both above, though in this case I would push for a "proper" PME and want to drop a rod at the MET with a connecting conductor capable of taking full diverted N current and then a bit probably, I would not dream of less than a 16mm tbh.

to be honest paul, i agree with getting PME, but as for the size of earthing conductor to the rod, it isnt neccesary to use a conductor of that size, unless your RA is very low ie, sub 4 ohms. all IMO

 

[netblindpaul]

johnboy,
The OP stated that the supply is already PME, though, to be best practice PME it should have a rod "at" the MET.
I would put the csa in to help keep the Z low, in the event of a fault current you cannot actually calculate the diverted fault currents so as you said IMO follow the Boy Scouts - Be Prepared!

 

[stuban]

Been working my way slowly through gn8 but skipping to 10.9 "potentially explosive atmospheres" (thanks for the steers) says "..a multiple earthed tn-c-s system are not considered suitable for locations where potentially explosive atmospheres exist"

It also addresses protection against static electricity.

Cheers netblindpaul that makes perfect sense. I'm all for belt-and-braces

Thanks all
Stu..

 

[johnboy6083]

to paul,

i understand what you mean, but the resistance of the conductor will be negligible compared to Ra. As for diverted currents, then ohms law has to apply? id never size a cable for earthing on its limits on a rod, in case ground conditions change for the better, and your fault curents increase.

 

[netblindpaul]

Been working my way slowly through gn8 but skipping to 10.9 "potentially explosive atmospheres" (thanks for the steers) says "..a multiple earthed tn-c-s system are not considered suitable for locations where potentially explosive atmospheres exist"

It also addresses protection against static electricity.

Cheers netblindpaul that makes perfect sense. I'm all for belt-and-braces

Thanks all
Stu..

Stu,
Doubt that you would have an Ex atmosphere around a domestic oil tank and DSEAR does not apply to domestic dwellings only workplaces anyway.
I realise a domestic premises becomes a workplace on times, however, I feel that DSEAR would still not apply!

 

[Spudnik]

Personally i would only bond the pipe if it is buried before it enters the building.

 

[topquark]

Comes under 411.3.1.2 only strictly necessary if it's extraneous is my understanding.

 

[IQ Electrical]

Comes under 411.3.1.2 only strictly necessary if it's extraneous is my understanding.

You're right if we're only looking at main protective bonding as opposed to some sort of functional or anti static bond at the tank end.

Once that rod is connected (maybe on advice of the tank manufacturer) the pipe automatically meets the definition of an extraneous-conductive part.

 

[stuban]

Netblindpaul point taken. In reality that applies more to dusty industrial atmospheres and petrol stations?

 

[netblindpaul]

^ Yep! Oh and a few others!

 

[stuban]

Jason, yes it is buried and not plastic coated so I would bond it as a normal service

That was what was throwing me. Surely the whole reason for bonding it goes against the reason for an earth stake at the far end - it's either at earth potential or not?

In which case one of these is not necessary(?)

Stu..

 

[netblindpaul]

What about OFTEC "regs"?

 

[IQ Electrical]

Jason, yes it is buried and not plastic coated so I would bond it as a normal service

That was what was throwing me. Surely the whole reason for bonding it goes against the reason for an earth stake at the far end - it's either at earth potential or not?

In which case one of these is not necessary(?)

Stu..

There's a difference between the potential of the distributors PME MET and the true earth that the pipe is buried in, saying that though, it's not a setup that I'd consider necessary on a domestic oil tank.

 

[stuban]

Fair points all of them.

I'll get some details of the tank - see if they spec it. And also have a chat with an oil heating engineer about the oftec regs.

Thanks a lot
Stu..

 

[truckster]

I do not understand this at all. Surely if you have PME then you bond the pipe as per usual but if the tank requires a stake for static this shuld be kept separate frm the PME by an insulated section of pipe. It seems risky sending potential earth faults towards a tank of oil. Just reread OP, its a plastic tank why would it need any sort of stake?

 

[IQ Electrical]

I do not understand this at all. Surely if you have PME then you bond the pipe as per usual but if the tank requires a stake for static this shuld be kept separate frm the PME by an insulated section of pipe. It seems risky sending potential earth faults towards a tank of oil. Just reread OP, its a plastic tank why would it need any sort of stake?

I'm reversing this discussion here, what we are doing in effect is connecting the oil pipework (and possibly the metallic fuel filler) to the neutral conductor of the installation.

The tank is then probably mounted on 'true earth'.

Maybe whoever initially proposed the electrode at the tank position was attempting to 'tie down' any potential difference between the suppliers MET and 'true earth'?

It reminds me of the advice from the Guidance Notes to insert an insulating link in copper pipework feeding an external tap on a PME system!

This is

 

[stuban]

IQ this is pretty much all I could guess at but since the pipe is buried and doesn't appear to be insulated would there be any benefit of the stake?

There were no specific instructions in the tank so in this case I'll bond where the pipe comes in have a chat with an oil engineer next time I see him.

Thanks all for the input
Stu..