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Topic Title: Metal oil tank in barn
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Created On: 18 May 2013 08:10 AM
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 18 May 2013 08:10 AM
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dickllewellyn

Posts: 1179
Joined: 19 March 2010

Morning all.

I've recently returned to a good (most of the time) customer of mine who lives in a beautiful old 17th century water mill.

As well as the mill which is his private residence with open gardens a few times a year, he has a small farm where he (or more realistically his workers) raise sheep and cattle. They are having a bit of a revamp, have a lovely big new green house, with fancy automatic irrigation etc, a work shop, and a tea shed to get the gardener s out of the house.

I've known about the job since January when they first started diggin things up and moving things round, but such is my relationship with the client that I rarely quote anything, just allow a few days for a job and turn up with a ream of day work sheets to sign.

I had the great privilege of seeing inside the barns and stables for the first time where the supply needs to come from for the new bits. The stables were connected onto the earth of the mill some 100M away, via a 10mm SWA buried under ground. The cable enters one corner of the stables to an old Wylex consumer unit which has since installation had a lovely metal oil tank put in front of it. With a bit of climbing it is possible to get to the consumer unit for isolation and to work on.

I was unhappy about this scenario, so I have made the stable block and all associated new out buildings a new TT supply, still using the existing 10mm, feeding a new consumer unit out the way of the oil tank. From here I took a submain to the back workshop to feed that, the green house, and the tea shed, all on lovely simple radials. I also took another cable back to re feed the existing old consumer unit with the instructions to the client that as we do more work in the future we will gradually move things over into the new one.

My concerns are mainly now with the oil tank. I don't believe it should be in the stables un bunded for a start, but I'm not hot on oil storage regs, so it could be acceptable. Does an internal oil tank need equipotential bonding? I know it won't be extraneous, but wasn't sure if oil regs would demand some green and yellow wire.

How about the electrical instalation? Would the area around the oil tank classify as zone 1? I guess the atmosphere isn't explosive, I know the oil itself doesn't burn that readily, but don't want to leave myself open to problems.

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 18 May 2013 09:01 AM
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perspicacious

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I'm wondering about which one of these 4 interlinked aspects of the existing cable:

10 mm2, 100 m, 15 A and 2.5% VD

is not the best starting point

Regards

BOD
 18 May 2013 12:36 PM
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davezawadi

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Joined: 26 June 2002

Hi Dick

The tank is presumably for heating oil, so static is very unlikely to be a problem. Is the pipe to the boiler metal or plastic? If plastic then its not extraneous and so no bonding required. It is not normal practice to bond oil tanks unless used for petrol or other light fractions. Watch the tanker driver filling one, if oil he just fills it, if petrol he is very careful with the earth bonding to the tanker before filling!

What is the overall supply, is it TT? Assuming yes then:
Now more tricky is why you want to run one TT supply from another, particularly as the mill probably has an excellent earth? Obviously you should check Ze but it is not necessarily a good idea to make lots of earth zones on one installation. If you feel that there is a risk of potential to the real earth at a remote location you can add more earth rods / mats etc, but it is not necessary to separate the earth connection, and if the tank has a metal pipe to another building, it is not possible to keep them separate when you add local bonding.

Remember that the reason for TT is partly to prevent additional risk from faults elsewhere putting voltages above earth on the exposed conductive parts of the installation for a significant period, because the earthing system has a relatively high resistance to actual earth. Any conductive connection between the zones causes an additional risk because you could contact both earth zones at differing potentials at once, and we normally deal with this by bonding together, so negating the multiple zones! So multiple RCDs are fine, multiple earth zones need careful thought and distance before implementation.

Are you sure the total load is small enough for 10mm as BOD says, workshops and kettles tend to take significant currents?
I hope that helps.

-------------------------
David
CEng etc, don't ask, its a result not a question!
 19 May 2013 01:04 PM
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sparkingchip

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This could be a good head scratching topic

Like davezawadi my first question is what the oil line feed pipe is made of and also the location of the boiler also what is the nature of the pipework to the central heating installation.

If the main house is PME earthed and the boiler is presumably controlled electrically from within the housefrom the house installation then anything connected with the boiler using cable or metallic pipework is part of the PME earthing system regardless of it's location. But if the boiler is in a outbuilding supplied from your TT installation with a programmer and thermostats in the house you could already have PME earth paths from your TT instalation.

Therefore if the metal tank is connected to the boiler in the house with a metal pipe it is already PME earthed by the metal pipe. If you put a bond in place to your TT sytem in the outbuildings it will then become effectively PME instead of TT.

If the oil feed pipeline is plastic then the tank may not need a earth as davezawadi said, but if the pipe is metal buried in the ground then changes to plastic as it gets to the house it definitely would need bonding at the barn end to the TT installation.

You could put a higher rated RCD 100mA or 300mA upfront of the whole installation and TT everything, then bond the tank locally in the barn without worrying about PME if it isn't already TT.

A bit of investigation of the current earthing of the heating system at the house end is called for with a check on what the oil feed pipe is made of is called for before proceeding.

If it turns out the oil tank is effectively PME earthed in a building with a TT earthing system it is not a major issue is it, unless you can touch the tank and exposed conductive parts of the barn at the same time?

Andy

Edited: 19 May 2013 at 01:44 PM by sparkingchip
 19 May 2013 01:09 PM
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sparkingchip

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It should be bunded, if it is on a concrete floor with a wall of dense concrete blocks rendered on the inner face was the standard method in the past, I'd suggest they consult their OFTEC registered installer and maintenance contractor for advise.

Andy
 19 May 2013 09:52 PM
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dickllewellyn

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Joined: 19 March 2010

Afternoon all.

Thanks for the replies. To address some of the comments and questions.

10mm is definitely a concern. I'm not too worried about the workshop load, that will only ever be used for charging batteries and sharpening chain saws etc. The greenhouses are a bit of a concern with heaters running, there is a likelihood they will also want a heater in the tea shed eventually, and of course the kettle. The length of the cable run is a guess, erring on the side of exaggeration, but a concern none the less. It is currently fed from a 40A MCB which I am planning to replace with a 20A on the assumption that a new cable run is out of the question.

The tank is indeed for heating oil. It used to be just outside the house, but was moved to the stable when they had some work done in the late Nineties before my time. The oil line is plastic, run inside a duct back to the house, which is indeed connected to a PME supply. The heating in the house is fantastic! There is a large thermal store tank with immersion heaters connected to a generator on the old water turbine, but it never gets used because the heating savings are lower than the fishing rates he can charge when the silt isn't all churned up, such is the bonus of owning banks of The Test (though kayakers among you will be aware there is dispute as to weather that means ownership of the river!). There is another thermal store tank in another part of the house which is fed from a wood burner with immersion backup. The pipework internally is all copper or steel. There are loads of old steel pipes about 4" diameter, the only controls being pumps controlled by a programmable room stat, and a shunt pump that runs on a pipe stat. There is a pair of boilers fed from the offending oil tank, but the boilers are in the house (or at least the barn attached on the side).

As for bunding, I think there may be issues of building compartments inside due to the stables being listed. The stables themselves are brick and flint construction, so presumably an additional internal wall of concrete block would be required.

I'm kind of hoping this will all be made much simpler before long, as the client is hoping to turn the stables (or part of) into a bungalow, bringing a new supply in from the pole. If this happens I can lose the submain from the house completely and load will not be an issue. Quite what will happen to the oil tank though is beyond me! In the mean time, if we can keep everyone safe, legal and happy, I can come back and collect my nice box of lamb from the farm, eat it, and get a good nights sleep without worrying! Chances are, the bungalow will never happen!

-------------------------
Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 19 May 2013 10:46 PM
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sparkingchip

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With a plastic oil feed pipe the green and yellow can stay in the back of the van, the tank is best left alone as it is indoors.

Andy
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