mice

[MdB]

... I only spent 25 years working on 11500 volt 50 Hz systems and 13800volt 60Hz systems carrying many 1000's of amps..............

I used to designed systems for measuring nV and fA over the range 0.0001Hz to 200kHz. Just a little bit down at the other end of the scale ;-)

 

[MdB]

Our earthing arrangements are that everything appears to be connected to the incoming electricity supply's earth in my cellar

Is that what's known as Protective Multiple Earthing (PME)?

PME (Protective Multiple Earth) also called TN-C-S (T = system uses earth, N = Earth supplied by supplier, C = combined neutral and earth, S = separate neutral and earth) are connections where the supplier provides the earth connection which is connected to multiple earthing points in their system. The multiple earth bit does not apply to earth connections within the consumers system which I think is a bit confusing.

With a PME system your supplier has to worry about the earth so your house system just connects to it and a local earthing spike is not required (and I believe not recommended).

Corrections to the above very welcome. There is plenty on it online if you want to look.

In Feltwell's case, I think I would want to confirm with the electricity supplier that it is a PME connection - there should be a label stating this but guessing by the age it isn't surprising that it is missing. It appears that the earth is connected to shield of the incoming connection as it would be in a PME connection but this may not be correctly earthed at the other end. When we moved in to our house, our incoming supply was so badly done, I couldn't even guess if we had a PME connection or not. I got the electricity supplier in to have a look, they agreed it was ancient and replaced it with a modern connection with a nice PME label for free. You may be able to get the same thing done.

 

[Penners]

I used to designed systems for measuring nV and fA over the range 0.0001Hz to 200kHz.

The only "word" I understood in that bit of Swahili was "Hz". As I understand it, 1Herz = 1 cycle per second. So, in the unlikely event that my maths is vaguely correct, 0.0001Hz means 1 cycle per 16.666' minutes! What the hell kind of alternating current is that?! :shock:

Feel free to use very short, simple words.

 

[MdB]

I used to designed systems for measuring nV and fA over the range 0.0001Hz to 200kHz.

The only "word" I understood in that bit of Swahili was "Hz". As I understand it, 1Herz = 1 cycle per second. So, in the unlikely event that my maths is vaguely correct, 0.0001Hz means 1 cycle per 16.666' minutes! What the hell kind of alternating current is that?! :shock:

Feel free to use very short, simple words.

nV = nano volts (0.000,000,001V)
fA = femto amps (0.000,000,000,000,001A)

I only came across one customer who was interested in making measurements at very low frequencies (long periods). He was stimulating plastic polymers if I remember correctly at different frequencies to find their natural resonances. Laterally, the polymer oscillated in the kHz (I think) but longitudinally it oscillation period was measured in hours. I may well have got some of that wrong but I do remember that our kit was capable of making the measurement.

Another theoretical example is a strain gauge fixed to a wall. It will produce a very low amplitude signal with a period of 12.42 hours. I'll let you work out what causes that.

 

[Penners]

nV = nano volts (0.000,000,001V)
fA = femto amps (0.000,000,000,000,001A)

I only came across one customer who was interested in making measurements at very low frequencies (long periods). He was stimulating plastic polymers if I remember correctly at different frequencies to find their natural resonances. Laterally, the polymer oscillated in the kHz (I think) but longitudinally it oscillation period was measured in hours. I may well have got some of that wrong but I do remember that our kit was capable of making the measurement.

Thanks, Mdb. I must remember that next time I have a plastic polymer I want to stimulate. :?

Another theoretical example is a strain gauge fixed to a wall. It will produce a very low amplitude signal with a period of 12.42 hours. I'll let you work out what causes that.

Might that be something to do with measuring movement in the wall? I always thought a strain gauge measured resistance in a wire - if the wall moves, the wire stretches, grows thinner, increases its resistance. But there is the tiniest, remotest possibility that I might be wrong. :wink:

 

[Zebra]

The gas meter is external, the pipework is connected to this earth just after it enters the house, so from what you say this is OK. The water pipe is plastic now - the old lead one is still there but is redundant, I was wondering just the other day if I should really bond this to the water pipes - what do you think? The water pipes are already bonded to the incoming electricity supply's earth, just after the stopcock. I can't see that it would do any harm, but I'm not convinced it's necessary either!
The only other service we have coming in to the house is BT, which I presume is not affected.
I've ran 10mm 3 core SWA cable from the garage underground and then through to the cellar. For the earth, I'm hoping that the earth core on the SWA plus the armour connection at the gland, connected to the earth block as shown above, will be OK.

All sounds fine. Not really any point in bonding the incoming lead pipework, since there's nothing connected to it that anyone's likely to touch. The reason for bonding is so that you can't get a shock off the electrical system whilst touching something (radiator, tap) at the same time which is naturally earthed by the fact that it comes in from the ground in metal pipes. Bonding the electrical earth to the gas/water "earth" stops there being a voltage difference between the two. So no harm in bonding the water pipes even though they don't go to outside in metal, but not strictly speaking necessary, any more than bonding any other piece of metal in the house that's not connected to anything else. You could go crazy, bonding every metal thing in sight!!

Power has been tapped off in the cellar for various things, it's a bit messy but it all seems logical and safe enough - just old, and has clearly evolved rather than being planned! A connection block is attached to the meter with some tails, and then various things are connected to that. It seems a good time to tidy this up.
The white RCD on the left is recent and is where an electric shower was connected. The cream coloured fused switch is redundant and was the old garage supply. I got a mini-consumer unit, intended for connecting a shower, free from my old work - I didn't know if that could be used to replace both those, with an 80 amp RCD on the incomer and a 40 amp MCB each for the feed to the garage and a feed to the shower. If not I'll do whatever the electrician advises.

Yeah, looks OK except needs a bit of a tidy up. Sounds like a good plan to replace the shower unit and old garage unit with a new RCD consumer unit. The electrician would have to work out whether 40A is suitable for the garage, should be fine on 10mm SWA but depends on the distance. I still have the big green book here if you want!!

See, you shouldn't have confessed to being an electrician, we're all going to want a bit of your knowledge now :wink: :lol:

Yeah look what I've started!

Re the PME thing, MdB's being a bit technical there :wink: (MdB was one of the people I had in mind when I said don't ask me technical questions, there are others on here more knowledgeable!) However, PME (otherwise known as TNCS) is as he says, where there's a combined earth and neutral at the incoming service head. The other system is TNS, which is what Feltwell has, where the earth is simply the sheath of the incoming cable. The only reason it makes any difference to a householder's installation is that the external earth fault loop impedance has to be lower on a TNCS (oh look at me, who's being technical now! I'll shut up )

 

[MdB]

Re the PME thing, MdB's being a bit technical there :wink: (MdB was one of the people I had in mind when I said don't ask me technical questions, there are others on here more knowledgeable!) However, PME (otherwise known as TNCS) is as he says, where there's a combined earth and neutral at the incoming service head. The other system is TNS, which is what Feltwell has, where the earth is simply the sheath of the incoming cable. The only reason it makes any difference to a householder's installation is that the external earth fault loop impedance has to be lower on a TNCS (oh look at me, who's being technical now! I'll shut up )

Too late Zebra, the questions are going to start coming in....

What's the physical difference between a TNS and a TNCS system as it comes into the house? From what I can make out in our system, the cable coming in from the pole outside has two conductors, the live centre and the neutral / earth sleeve. Is it in fact three core - live centre, neutral around it then earth around that?

 

[Zebra]

What's the physical difference between a TNS and a TNCS system as it comes into the house? From what I can make out in our system, the cable coming in from the pole outside has two conductors, the live centre and the neutral / earth sleeve. Is it in fact three core - live centre, neutral around it then earth around that?

Oh well, in for a penny in for a pound! I don't really know much about supply conductors, never having installed one, or even taken one apart. All I know about is what happens once it's terminated inside the house. So...
TNS is where the earth in the house is joined to the sheath of the incoming cable. Normal for older installations.
TNCS (PME) is where the earth of the house is joined to the neutral conductor of the incoming cable - you normally see it going into the side of a box at the service head. There should be a label saying it's PME, but well, there might or might not be. What happens to the combined neutral/earth conductor once it goes back to the substation I did learn in college but can't remember!
If what you have is an overhead supply, then it's very likely to be TT - different again, and common in rural areas. With TT there is no earth connection supplied by the electricity supplier and you have to make your own arrangements with earth rods/spikes.

What is your earth bar / the main earth from your consumer unit connected to?

 

[Penners]

We should rename this thread, "Do Androids Dream of Electric Mice".

 

[Feltwell]

Yeah, looks OK except needs a bit of a tidy up. Sounds like a good plan to replace the shower unit and old garage unit with a new RCD consumer unit. The electrician would have to work out whether 40A is suitable for the garage, should be fine on 10mm SWA but depends on the distance. I still have the big green book here if you want!!

Thanks a lot Zebra, for that and all the other advice. If nothing else it's good to know that my RCD mini-consumer unit idea isn't a totally daft one, and suggesting it to a visiting electrician is not going to instantly give rise to thoughts of "Christ, this bloke really is a complete numpty, what the hell else has he done?" :wink: I've got all the bits, they didn't cost me anything, so if he/she can use them then great, if not then no loss.

No need for the green book I think - it's not a long run (about 13m), only about 4m of that is underground, there will be another RCD plus a 32 and a 6 amp MCB on t'other end in a garage consumer unit, so I think 10mm will be OK - the calculator I used reckoned that 6mm would just do it, so I went up a size just to be on the safe side. I flaming hope so - it was enough of a struggle getting that cable through, let alone anything meatier!

 

[MdB]

TNS is where the earth in the house is joined to the sheath of the incoming cable. Normal for older installations.

With TNS the S means split neutral and earth so this implies that the sheath is earth and there is an additional neutral inside the cable somewhere.

TNCS (PME) is where the earth of the house is joined to the neutral conductor of the incoming cable

Which is also the shield of the incoming cable. i.e. the incoming cable sheath is both neutral and earth.

If what you have is an overhead supply, then it's very likely to be TT - different again, and common in rural areas. With TT there is no earth connection supplied by the electricity supplier and you have to make your own arrangements with earth rods/spikes.

Not in our case, the three cables on the poles are two phases and earth/neutral (according to the guy wiring up our house so I hope he knew what he was talking about). This enables us to have a TN-C-S (PME) system and we have the label on our supply.

 

[Zebra]

I'm not sure. As I said, I don't really know much about supply cables. As long as I get a good reading for the earth on my test meter, that's good enough for me.

Anyway, these mice.......

 

[Feltwell]

Anyway MdB, it's not the earthing arrangements or your incoming supply that you need to worry about, it's those dodgy sockets and switches that you fitted.............

 

[MdB]

Anyway MdB, it's not the earthing arrangements or your incoming supply that you need to worry about, it's those dodgy sockets and switches that you fitted.............

Guess who had a socket make a wonderful sparking like noise a couple of days ago... Thought it must have been a dodgy connection in the back but all was fine. My guess is it was a spider or a fly inside that workings but its probably easier just to blame the geezer who I bought it from... :twisted: :wink:

 

[Feltwell]

He's a slippery character, you won't get any joy from him.

Remove the module from the back of the socket if you want to check - they are simple enough inside, though I can't remember if that one is held in place with screws or rivets, the earth bar may well be riveted across the back of the module. If it's rivets or if you're not sure then call 0845 194 7584 and have a moan, they should give you a freepost address to return it to and send you out another. Just say you got it from your local electrical wholesaler, though they are unlikely to ask.

 

[MdB]

Another theoretical example is a strain gauge fixed to a wall. It will produce a very low amplitude signal with a period of 12.42 hours. I'll let you work out what causes that.

Might that be something to do with measuring movement in the wall? I always thought a strain gauge measured resistance in a wire - if the wall moves, the wire stretches, grows thinner, increases its resistance. But there is the tiniest, remotest possibility that I might be wrong. :wink:

Yep, all the walls in the country are moving and flexing a little (at 0.000022Hz) and with a sensitive enough detector it is possible to pick this movement up. A little extra clue - you also pick this signal up if you measure the movement of the bedrock.

 

[LadyArowana]

Yep, all the walls in the country are moving and flexing a little (at 0.000022Hz) and with a sensitive enough detector it is possible to pick this movement up. A little extra clue - you also pick this signal up if you measure the movement of the bedrock.

We are not about to embark on another sustainable energy debate are we?

 

[MdB]

Yep, all the walls in the country are moving and flexing a little (at 0.000022Hz) and with a sensitive enough detector it is possible to pick this movement up. A little extra clue - you also pick this signal up if you measure the movement of the bedrock.

We are not about to embark on another sustainable energy debate are we?

Interesting idea... Hmmm - if you were to cover the country in piezo then there is a possibility of generating an insignificantly small amount of green electricity. I wonder if there is a more efficient way.... (there is actually and it is already being used but that would give the game away)

 

[Penners]

you also pick this signal up if you measure the movement of the bedrock.

Bedrock?!

Cornwall may be hewn out of solid granite, but East Anglia was built by piling up gravel, sand and clay. Your detector would probably pick up the wobbling from some farmer's ploughing, up in Norfolk.

 

[LadyArowana]

Yep, all the walls in the country are moving and flexing a little (at 0.000022Hz) and with a sensitive enough detector it is possible to pick this movement up. A little extra clue - you also pick this signal up if you measure the movement of the bedrock.

We are not about to embark on another sustainable energy debate are we?

Interesting idea... Hmmm - if you were to cover the country in piezo then there is a possibility of generating an insignificantly small amount of green electricity. I wonder if there is a more efficient way.... (there is actually and it is already being used but that would give the game away)

*Cough* I was making an obtuse reference to that.

I knew I shouldn't have had the blonde dye job, everyone deducts about 50 points off the IQ.