Understanding the electricity supply to my house (pole, underground cable, meter box, etc...)

[barkymalarkey]

Partly cos I like to know everything... Partly because I'm a geek... And partly because we're going to have some electrical work done on the house which has made me curious.... I'd like to know more about the feed to my house...

The power lines come to us through the garden and reach a pole with transformer. From there the supply comes to us underground. Because the ground is lower at the back of the house, you can see the main cable going into the wall of the house. I'm guessing it then travels under the house to the meter box which is on the front of the house. The meter box has two fuses. The house has three consumer units. As we're having the consumer units changed and a partial rewire amongst other things, I'm not too worried about comments on the photos of the consumer boxes (but feel free to make them if you wish)...

... What I am more eager to know is anything you can tell me about the pole, transformer, underground line and meter box. Some of the things of particular interest...:

1. Earth/ground. How can I tell what the set up is for that? A sparkly told me there was a supplied earth, but there's only two overhead cables. But there are three cables running down the side of the pole and going into the ground. 

2. Earth/ground - is there any way to test it? (I'll be running a music studio in the house which I've been told needs a low impedance path to ground)

3. I'd love to know what's what on the pole and how it works, so any explanations or links would be fantastic.

4. The diagonal "cable" with the yellow cover coming from the pole. Is that purely just a support or does it have something to do with the earth/ground?

5. Why two fuses in the meter box?

6. Can anyone tell me roughly how old those two fuses are?

7. How often are the power companies maps wrong? I was in touch with them today and they sent me a map which shows the cable running underground from the pole to the front of the house. But I can see a power line going into the back of the house (see photos). Odd...

8. Anything what you want to enlighten me with!

I've got no broadband at home and so it takes an age to upload anything. I'll therefore link to a folder on Google drive that has all the photos. Let me know if that's a problem and I'll upload the photos to the site when next at the library.

Sorry for such a long thread. Any thoughts most welcome. Cheers 

https://drive.google.com/folderview?id=1-5SRqpupjqCplW9hDkE06pA9UjCL1hU9

 

[phil d]

OK, lets take your questions one at a time, there are 3 types of earthing system, TNS, TNCS, AND TT, in a TNS system the earth is fed from the supplier as a totally separate entity, usually on a lead sheathed cable, basically inside the lead sheath you have 2 cores, Live and Neutral, the lead is the earth core. Then you have TNCS, in this system the earth is taken off the neutral at the incomer, or service head, the black box with the fuse in it before the meter. Then you have TT, in which case the earth is derived from a rod driven into the ground, these systems normally have a higher earth loop value and are protected by an upfront RCD rated at 100mA.

In your case I suspect it is a TNCS system, also known as PME, protected multiple earthing. To measure what is known as the EARTH LOOP IMPEDANCE, we use a loop tester, there are maximum values of loop laid down not only by BS7671, wiring regs, but also by the DNO the people who own the cables that bring electricity into your home. For a TNCS system this is 0.35 ohms and for a TNS it's 0.80 ohms, it's a lot higher for a TTsystem, normally 100 ohms. If the value of earth loop is too high then in the event of a fault to earth there may be insufficient current flowing to trip a breaker and this can result in cables overheating and catching fire. I suspect that your system is a TNCS.

The large box on top of your pole is what is known as a Pole Mounted Transformer, the 2 lines coming in overhead will be running at 11,000 volts normally and the transformer drops this down to around 230/240V to feed your property. The grey box below it is a fuse, this is to protect the cable from the transformer, or TX as they are known in the trade, and also to allow for isolation for maintenance.

The yellow 'cable' running from the pole is actually a steel rope, known in the trade as a 'pole stay', or Back Stay on the end that goes into the ground will be what is termed a 'fluke set', this is basically like a small folding boat anchor and prevents the cable from being dragged back out. It is fitted to counteract the strain on the pole caused by the overhead cables, this is known as the 'pull on pole' Lets imagine that the cables to the TX are coming in from the left and are 50 mtrs to the next pole, the strain (pull on pole) will be high enough to possible cause the poles to lean over time, so by putting the 'pole stay' on and having it from the opposite direction it counteracts the strain of the overhead cables. The yellow sleeve is simply a plastic tube to make the cable more visible and prevent people from not seeing it.

Inside the service head, or 'cut out' as we call it there will only be one fuse, this will always be in the live side of the supply, many years ago it was common practice to put a fuse in both the Live and Neutral wires, but this was stopped as it was possible in fault conditions for the fuse in the Neutral side to blow, leaving the live intact, this could lead to a very dangerous situation.

The 2 boxes, one white, one brown, are actually mainswitch and fuse units, I suspect that these will be feeding the other consumer units within the property. It must be a fairly old installation, say 1970's or 1980's judging by those boxes, also there is no RCD protection on them to protect the cables going from them to the main consumer units within the house, which unless those cables are either armoured, or protected by a metal conduit, or run on the surface and clearly visible, is a requirement of the current version of BS 7671.  the main fuse is inside the black box, or Service head, below the meter.

The  block on the top of the pole stay cable is a large insulator, this is to electrically isolate the pole stay cable so that in the event of one of the 11kv cables coming adrift (it does happen) the pole stay cable will not become live.

It's rather interesting how those anchors are fitted, the fluke set is hinged at it's lower end and folded up ( think of an umbrella) the metal rod is driven into the ground and then when it is all the way in, the bar with the eyelet in the top is rotated slightly, this releases the flukes, a slide hammer is then attached and the steel rod is dragged back a little way, this has the effect of spreading the flukes outwards (opening the umbrella) and locking the thing firmly in the ground, once in they generally have to be dug out to remove them.

Electricity board cable maps are generally pretty accurate, however this doesn't mean that they cannot be wrong, it's always possible that a cable has been replaced in a position different from the original and the drawings not updated, which is why, before digging anywhere where there may be a buried cable, or indeed any service, e.g a gas main it is as well to contact the provider of that service to ask about it's location, some will come out and locate it's route for you, others simply supply a drawing, when we dig, we normally use something called a CAT (cable avoidance tool) and this will detect a buried cable in much the same way as a metal detector finds buried metal, they are reasonably easy to use, but it is possible to misinterpret the readings occasionally if you are not experienced in their use.

Hopefully this has given you a bit more of an understanding of how the system works, however I'm sure if I've missed anything then one of the other lads on here will fill in any gaps. best wishes, Phil 

 

[ProDave]

Looking in your meter box I am pretty sure that is a TNC-S supply, the incoming cable being concentric (think of it as a very overgrown coax cable)

I don't know why you have 2 switch fuses, they must be feeding off to two separate places perhaps one to each of your present consumer units?.  You need these because your consumer unit is some distance from the meter box. These can remain and continue in use as long as upon inspection and testing they are in good contition.

HOWEVER the meter tails feeding OUT of your electricity meter and into your switch fuses (the thick red and black wires) really need to be replaced with bigger ones.  Get your electrician to do that when he replaces your consumer unit.

The electrician doing the consumer unit change should do a whole lot of testing in the process and will identify anything that needs putting right. It is quite likely earth bonding in the house will need updating etc.

 

[barkymalarkey]

What can I say... Such amazing responses! Can't tell you how much I appreciate those. Sorry it's taken me a while to return. I had to travel back home due to family illness and so have been away for a week. 

I'll doubtless have other questions but while I get my head around all of that info I just wanted to say a big thank you.

Speak soon

Cheers

 

[barkymalarkey]

I'm back! First I just want to say thanks again for those replies . They were so helpful and really above and beyond! Cheers

Ok, so I do have a few more questions and help with any of them would be most appreciated.

A) There's some debate over the black cable going into the back of the house and whether that's the mains supply from the transformer. I think it is but others think not. What do you guys think (see pics labelled 04 and 05; link in first post)? The diameter of the swa is around 18mm. The black sheath says 600 - 1000V. It seems to be exactly the same as what comes into the meter box cutout (bottom left). There's no sign of the cable inside the property. In fact, where it enters the wall is roughly 30cm below the current floor level. 

B) In the meter box, someone commented that the cutout ground connection block is corroded and I should maybe speak to the DNO about replacing it. What do you guys think?

C) @phil d Would you suggest me asking my electrician to change the two switchfuses for new ones with protection so that the cables from the meter box to c/units are safe?

D) The two switchfuses in the meter cupboard... The white one controls the c/unit in the lobby. The black one controls the other two c/units, IE the understairs and cow shed mezzanine ones. The consumer units do work independently though. If you turn one off the others still work in all configurations. 

E) Looking at the wooden pole in the garden again there is one wire coming down on one side and two on the other side, and all of them go down and into the ground. Why three??? I'll take a photo and upload...

Thanks a million for all the help.

 

[barkymalarkey]

I've uploaded the additional photos to the same folder as the other photos. Link above but here it is again to save scrolling:

https://drive.google.com/folderview?id=1-5SRqpupjqCplW9hDkE06pA9UjCL1hU9

The additional photos are labelled 3h, 3i, 3j and 3k. They show the the cables coming down the pole and into the ground.

Cheers

 

[Murdoch]

Apologies for might seem a daft question but why are doing all this ?

 

[barkymalarkey]

I just like to understand these things. It's interesting...

But also, if I learn things like the wiring from the switchfuses to c/units is unprotected (for example), I'll know to have something done about it.

 

[Sharpend]

Wow, I bet you're a pleasure to live with, are you like this with everything? 

Most people just wait until something goes wrong before investigation. 

 

[Rob.]

Wow, I bet you're a pleasure to live with, are you like this with everything? 

Most people just wait until something goes wrong before investigation. 

Whats wrong with wanting to understand how things work?  

 

[barkymalarkey]

Yes, I'm like this about most things. And no, I'm not a pleasure to live with; not even with myself! I just can't help it... We all have our foibles I guess and this is one of mine.

If all was well with the electrics then I'd be doing as you say and would wait until there was a problem. But it's not... Part of the house needs a rewire, the 3 c/units need replacing and possibly reconfiguring, and the cowshed needs gutting at a later date at which point the electrics in there will need redoing as well. Plus the cowshed will have a music studio which might need particular requirements.

Hence all the questions as I want to make sure things are done right for now and for the future when the cowshed is done.

Update: Thanks Rob.

 

[Sharpend]

Whats wrong with wanting to understand how things work?  

Nothing, I'm mad for understanding but then I'm in a trade so I guess that comes with the territory, but if I wasn't then I'm not sure if I'd be that interested. It was the love of understanding that brought me into this trade and along the way I have learned many other skills through wanting to understand, although it's fair to say that I do use them all regularly. 

I guess what I'm saying is that if you are that inquisitive then isn't natural that you'd want to work in that environment? 

 

[Rob.]

Nothing, I'm mad for understanding but then I'm in a trade so I guess that comes with the territory, but if I wasn't then I'm not sure if I'd be that interested. It was the love of understanding that brought me into this trade and along the way I have learned many other skills through wanting to understand, although it's fair to say that I do use them all regularly. 

I guess what I'm saying is that if you are that inquisitive then isn't natural that you'd want to work in that environment? 

Well we don't know what the OP does for a living.

I know my interest in how things work got me into engineering, but you an I both know, being interested an researching to fill curiosity is completely different when you have to do it for work.

 

[Sharpend]

Well we don't know what the OP does for a living.

Fair comment. 

I know my interest in how things work got me into engineering, but you an I both know, being interested an researching to fill curiosity is completely different when you have to do it for work.

Absolutely. 

 

[barkymalarkey]

So now that we've cleared all that up and determined that I'm over inquisitive and/or half mad.... Any thoughts about my questions?!?

 

[Murdoch]

So now that we've cleared all that up and determined that I'm over inquisitive and/or half mad.... Any thoughts about my questions?!?

Hum, write a specification of what you want now, include outlines of what you think you want to do in the future and make sure people who bid for the work all quote for the same ....

Then you should avoid the conversation with a spark who says " I wish you had told me this earlier"

 

[phil d]

Hum, write a specification of what you want now, include outlines of what you think you want to do in the future and make sure people who bid for the work all quote for the same ....

Then you should avoid the conversation with a spark who says " I wish you had told me this earlier"

Agree with that one, it must be annoying for the customer, but it's even more annoying for me, you get the job, think you've finished it then the customer says, "oh, the other guy said he was going to fit a couple of outside lights as well"

Then it leaves you wondering, did the customer genuinely forget the lights, highly unlikely, or are they trying it on, probably, which is why, for years now I've point blank refused to work for a certain group of people, mind you I'm honest, I tell them I won't work for them because I've had loads of them try and rip me off, it's not like I make excuses.

 

[SPECIAL LOCATION]

Then it leaves you wondering, did the customer genuinely forget the lights, highly unlikely, or are they trying it on, probably, which is why, for years now I've point blank refused to work for a certain group of people, mind you I'm honest, I tell them I won't work for them because I've had loads of them try and rip me off, it's not like I make excuses.

Excluding any very simple tasks...

I can't believe anyone wouldn't do a written contract with a schedule of work to be done before agreeing to do any work...

Stops ALL confusion/rip-offs before the job starts?

:C

 

[Murdoch]

Excluding any very simple tasks...

I can't believe anyone wouldn't do a written contract with a schedule of work to be done before agreeing to do any work...

Stops ALL confusion/rip-offs before the job starts?

:C

And more importantly its easy to identify the "extras" that the client asked for during the job which weren't included...........

I tend to write a "draft" spec , send it to the client, get them to review and comment and then do a price............

Locked to prevent being resurrected (again)