Just Moved And Learned What A Tt Earth Is, Some Questions!

[curious-one]

Hi all,

I've always taken the electric supply for granted, RCDs, Earth etc..  but have moved to a house with an old Wylex wooden backed fuse box and a separate whole house 100ma RCD.  I noticed an earth rod in the front garden so have spent a few days reading up.  I'm used to multimeters for basic testing so have been having a dig.  First up, I (and you'll probably laugh here) I turned off power to a socket circuit and rigged up a long extension cord to take out of the house to where the rod is, I then tested continuity and found it ok to the end of the earth wire but none elsewhere on the rod.  Looked into this, went and bought a new earth rod immediately and stuck it in the ground, trimmed the old discoloured wiring and attached it to the new rod (power was completely off whilst I did this).  I now have continuity between my socket earth and the rod.

After doing this I became curious and measured voltage between a radiator and a light switch screw and found 0v (as I expected as all earthed together, continuity between objects confirmed), but if I test voltage by placing one probe on my finger and the other on the earth I find between 11 and 15v depending on when I do it (I've done it several times)  but if I put a 1k resistor in place it drops to 0.01v, so no real power.  I also found a voltage of about 4v between Neutral and Earth at the back of a socket (live at the time, I used a class 3 meter with short tipped probes), is this normal?  As I say I never took a reading before in any previous house as I did what most do and just used it, but finding out about a TT earth and looking in deeper has got me a) curious, b) in need of a change to a CU with 30ma RCD protection and c) a little freaked out that I had no earth in the first place.  Anyway, thanks for reading.

 

[Lurch]

I've always taken the electric supply for granted, RCDs, Earth etc..  but have moved to a house with an old Wylex wooden backed fuse box and a separate whole house 100ma RCD.  I noticed an earth rod in the front garden so have spent a few days reading up.  I'm used to multimeters for basic testing so have been having a dig.  First up, I (and you'll probably laugh here) I turned off power to a socket circuit and rigged up a long extension cord to take out of the house to where the rod is, I then tested continuity and found it ok to the end of the earth wire but none elsewhere on the rod.  Looked into this, went and bought a new earth rod immediately and stuck it in the ground, trimmed the old discoloured wiring and attached it to the new rod (power was completely off whilst I did this).  I now have continuity between my socket earth and the rod.

Whilst this sounds great that's only really half the job of doing the testing correctly.

I also found a voltage of about 4v between Neutral and Earth at the back of a socket (live at the time, I used a class 3 meter with short tipped probes), is this normal?

It's not unusual.

As I say I never took a reading before in any previous house as I did what most do and just used it, but finding out about a TT earth and looking in deeper has got me a) curious, b) in need of a change to a CU with 30ma RCD protection and c) a little freaked out that I had no earth in the first place.  Anyway, thanks for reading.

I would look at getting this done sooner rather than later so that the earthing arrangements can be tested & verified as part of the CU change process. Judging by the age of the CU then it is likely that there will be more work required.

 

[curious-one]

Thanks, yes I was reading up on testing earth and sadly I can't justify the several hundred pounds for a fluke or megger, which is in a shame as it's got me in a gadget mood.  I'm waiting for a call back from a local electrician to come and do proper testing as calculating Ze looks a bit tough with a multimeter and I can't see how to get Zs under load without getting fried by my meter (can you tell how much reading I've done!)

 

[steptoe]

1st things first

make absolutely sure your electrician understands TT systems

this is NOT to be taken lightly, not all 'electricians' are what they seem,

you do not measure Ze on a TT system, Ra is more important,

then Ze,

measuring continuity with a cheapish multimeter may be fine, measuring PD is something completely different and your meter may be showing phantom voltages.

 

[curious-one]

Thanks, and yes I gather not all are what they seem lol, I had a bit of an argument with one over consumer units, he swears blind they must all be steel after January 1st, but I want plastic as I have a TT Earth and supply side fault could electrify my house, so explained I wanted plastic but was happy to have it installed inside a steel box.  He said that wouldn't comply with the regs, at which point I loaded up the specific text from MK's site which confirmed plastic in a metal housing is also acceptable!

And yes my meter does pick up phantom (or induced I think) voltages, there are two disconnected twin and earth wires to a burned out socket in the house (this happened under previous owners) and if the socket circuit is off (yes this house has only 1!) then I get a reading of 0v across all terminals.  If I switch the breaker on for the house sockets, I pick up 12v between line and earth on that disconnected cable, but if I put a 1k resistor across the wires and then test it comes back at 0v, so phantom/induced voltage it seems to be.  Hopefully one is coming today and I can get him to IR tests and R1 and R2, then do Ra and confirm nobody is going to die in the shower (I've had it switched off since finding some faults and won't let anyone use it).

Thanks for your replies all.

 

[Andy™]

your sparky is correct in that from next year they must be steel. its a regulation many of us do not agree with for many reasons, including the simple fact that it has already been proven not to work for its intended use

 

[thebight]

your sparky is correct in that from next year they must be steel. its a regulation many of us do not agree with for many reasons, including the simple fact that it has already been proven not to work for its intended use

Really? Pretty sure OP is correct and you can still use plastic consumer units provided they are installed in a suitable non-combustible enclosure.

EDIT: Yep, OP is correct, confirmation from the IET

 

[Lurch]

your sparky is correct in that from next year they must be steel. its a regulation many of us do not agree with for many reasons, including the simple fact that it has already been proven not to work for its intended use

No he isn't, they do not all need to be steel. They can be plastic in a non-combustible enclosure as the OP said. They also don't need to be steel if they aren't on an escape route/under the stairs etc (although in most domestic properties pretty much everywhere is an escape route).

 

[Sidewinder]

No he isn't, they do not all need to be steel. They can be plastic in a non-combustible enclosure as the OP said. They also don't need to be steel if they aren't on an escape route/under the stairs etc (although in most domestic properties pretty much everywhere is an escape route).

Not quite strictly true Lurch.

Yes they can be plastic in a non-combustible enclosure anywhere.

However unless they are in a non-combustible enclosure then the CU itself must be non-combustible.

This has been taken to be steel by "the powers that be".

There are no exceptions for escape routes or not the reg is all encompassing and covers domestic dwellings and their attached outbuildings, certainly if there is a possibility of the spread of fire from the outbuilding to the dwelling.

The only escape route thing is for an existing plastic CU in an escape route in a domestic, is recommended as a C3 on an EICR by the Electricity Safety First best practice guide.

 

[Lurch]

Not quite strictly true Lurch.

Yes they can be plastic in a non-combustible enclosure anywhere.

However unless they are in a non-combustible enclosure then the CU itself must be non-combustible.

That is exactly what I just said, and also what the OP said.

There are no exceptions for escape routes or not the reg is all encompassing and covers domestic dwellings and their attached outbuildings, certainly if there is a possibility of the spread of fire from the outbuilding to the dwelling.

The only escape route thing is for an existing plastic CU in an escape route in a domestic, is recommended as a C3 on an EICR by the Electricity Safety First best practice guide.

I'll agree on the dwelling part as I can't find what I have read that seemed to imply that escape routes/under stairs had to be non-combustible, elsewhere was open to be risk assessed.

Amendment 3 doesn't cover outbuildings though.

 

[curious-one]

Always fun to begin a discussion.  The current Fusebox is just outside my bedroom, above the door.  So it's an escape route for me if a fire broke out in the night, though having said that there is also a window out onto the roof of the garden room so I could escape that way without breaking my ankles.  I must say reading about the CU fires has got me thinking, I've never even considered checking tightness of the terminal screws in previous houses, just gone down the thought line of "A qualified spark installed it, so it must be fine".  Having read on a few electrician forums now that a fair few have gone back to units they know they did correctly and found loose connections shows just how much change electromechanical force can cause.  Also Lurch was right on finding more faults before a CU, I was checking the switches and found very old rubber insulated wiring so I'm changing that to 1.5mm twin and earth before I get a spark in, no point bringing one over to point out something I can already see.

I must confess though, all this has made me think about all the light fittings and sockets I've just casually changed over the years without a thought for circuit resistance (like thousands of people I guess).  I only began doing it after a childhood of watching my dad take out all the lights for 4 hours every time something needed changing so figured it out as a teenager and done it ever since, and thank god for live dead live testing.  My grandmother asked me to change a collapsed socket for her once and confirmed she'd turned the power off, luckily I always test myself and discovered she'd just turned off the shed power.  Thankfully I've never been electrocuted, so I'll stick with my current regime of triple checking everything and hopefully this mess will get sorted very soon.

 

[Lurch]

I must say reading about the CU fires has got me thinking, I've never even considered checking tightness of the terminal screws in previous houses, just gone down the thought line of "A qualified spark installed it, so it must be fine".  Having read on a few electrician forums now that a fair few have gone back to units they know they did correctly and found loose connections shows just how much change electromechanical force can cause.

The problem is not that there is anything wrong with (real) electricians methods or the consumer units themselves but that the current crop of electricians are not actual electricians, more semi-skilled/retrained amateurs.

Also Lurch was right on finding more faults before a CU, I was checking the switches and found very old rubber insulated wiring so I'm changing that to 1.5mm twin and earth before I get a spark in, no point bringing one over to point out something I can already see.

I'd leave it and get the testing done first TBH.

Why are you using 1.5mm²? Is the rest of the circuit in 1.5mm² or is there a volt drop issue on that particular switch drop? What is the CPD for that circuit?

 

[Sidewinder]

...

Amendment 3 doesn't cover outbuildings though.

Well it does, but 421.1.201 does not include the metal cu bit for outbuildings, I'll agree!

However, you will find that "industry" guidance, and that which will come from the IET shortly will include attached outbuildings.

It should have been written into the reg tbh.

 

[Lurch]

However, you will find that "industry" guidance, and that which will come from the IET shortly will include attached outbuildings.

Hmm, the guidance I read from the IET said not, we even had a thread on it the other week. If the IET are going to flip-flop and change their minds then I am going to ignore everything they say.

Consumer units in outbuildings or on the outside of a building

Regulation 421.1.201 uses the term ‘premises’. The question could therefore arise: do the requirements of the regulation apply to a consumer unit or similar switchgear assembly within an outbuilding such as a garages or shed, or mounted on the outside or a building?

Some dictionary definitions of ‘premises’ are ‘a house or building, together with its land and outbuildings’ and ‘the land and buildings owned by someone’.

However, Regulation 421.1.201 was principally introduced to cover the interior of a household building and any garage or other outbuildings integral, attached, or in close proximity to that building.

Doubt could exist about whether or not a particular outbuilding could reasonably be considered to be in ‘close proximity’ to the household building. A way of resolving this might be to make a judgement of the likelihood that fire originating inside the enclosure of a consumer unit or similar switchgear in the outbuilding might lead to the outbreak of fire in the household building or in any outbuilding integral or attached to it. Relevant factors to consider about such an outbuilding might include whether or not that building or its expected contents are highly combustible.

Regulation 421.1.201 is not intended to apply to a consumer unit or similar switchgear assembly that is not within a building, such as a consumer unit mounted outdoors on the outside of a building.

Source: http://electrical.theiet.org/wiring-matters/55/consumer-units/index.cfm

 

[Sidewinder]

Lurch,

That pst extract from WM, is exactly what I said earlier, and what I said in the last thread.

It is also what is in section 2.2.6 of the OSG.

I can't give you a page number, because my SOG doesn't have page numbers, which is a right pain!

421.1.201, applies within the dwelling and it's attached outbuildings or those close enough to allow propagation of fire to the dwelling.

Which is what I said and what your quote from WM is saying above, so quite how you can say that the guidance is changing I don't know.

There is more guidance due out shortly which will reinforce this I have been told.

 

[Lurch]

I think what is happening here is we are interpreting the same thing differently. When I said guidance is changing I was comparing my interpretation to yours.

 

[curious-one]

The CPD is 6A and the old wiring looks to be 1.5mm, the house is weird and needs a fair amount of cable to cover it (it looks massive from the outside, but not inside) so I suspect volt drop could crop up, so I thought I'd go for my 'overkill is underrated' approach to fixing it.  As for outbuildings, I will be running power to the new shed at some point with some SWA, but the shed is about 200ft away so fire spread shouldn't be a worry, as the building is wood I also doubt it'd matter whether the CU was plastic, metal or a firelighter, anything would get hot enough to ignite the wood in the event of a fault I suspect!

P.s. as a thought (and don't worry, a real electrician will be asked to test this!) but although I can't use a standard DMM to test resistance under load, could I not test voltage between L and N at the fuse board, then at the last light on the radial with all of the lights on, then work out resistance under load as I know the wattage of all of the lights?

E,g. lets say I get a perfect 240v at the fusebox, then I get 236v at the last radial (I haven't done this yet, these are just illustration figures) and I know I've got, say, 5x60W bulbs on the circuit all powered giving a total of 300W would be 1.25A draw.  That would give me a resistance figure of 192 ohms, as it's a resistive load I've got no PFC to worry about, or have I misunderstood the test?

 

[Lurch]

The CPD is 6A and the old wiring looks to be 1.5mm, the house is weird and needs a fair amount of cable to cover it (it looks massive from the outside, but not inside) so I suspect volt drop could crop up, so I thought I'd go for my 'overkill is underrated' approach to fixing it.

Well, I wouldn't use 1.5mm² as it makes terminating into downlighters and light fittings awkward, and also costs more. I sort of agree with the overkill is underrated thing to an extent but 1mm² cable is usually perfectly fine and the disadvantages outweigh the advantages in most cases for a standard domestic lighting circuit.

P.s. as a thought (and don't worry, a real electrician will be asked to test this!) but although I can't use a standard DMM to test resistance under load, could I not test voltage between L and N at the fuse board, then at the last light on the radial with all of the lights on, then work out resistance under load as I know the wattage of all of the lights?

E,g. lets say I get a perfect 240v at the fusebox, then I get 236v at the last radial (I haven't done this yet, these are just illustration figures) and I know I've got, say, 5x60W bulbs on the circuit all powered giving a total of 300W would be 1.25A draw.  That would give me a resistance figure of 192 ohms, as it's a resistive load I've got no PFC to worry about, or have I misunderstood the test?

What test are you trying to simulate here?

 

[curious-one]

I'm trying to see if I can do a basic test of resistance under load which I believe is part of the standard tests an electrician would do before energising a circuit.  I can do a simple dead test by connecting L and N with a terminal block and testing round trip from the last point, again same for L and E and then N and E, but if I remember my school days right (and it's possible I don't!) the resistance figure will change once a load is applied, so the dead test is little more than a continuity test with a DMM, but the resistance under load is what sets things on fire.  I'm basically trying to see if I can cover some basic tests so when someone comes out, I can hopefully make life much easier (and less costly, I'll admit, car broke the other day so I'm poorer than I was!) in the hope they can run their fluke or megger across everything and all will be well.  Thanks.

 

[Sidewinder]

Lurch,

How are you interpreting the existing IET guidance in WM, the OSG etc then, as the way I read it, it includes attached outbuildings, and any in close proximity that could cause the fire to spread to the main dwelling.

It almost uses those exact words, so what other interpretation are you putting on it, because I pretty much can't see any?

Are you taking the reg to literally mean only within the dwelling house itself, and you are excluding all attached outbuildings such as an attached garage?