TT vs TN-C-S

[HDRW]

It comes in underground and is coated in what looks like bitumen. There is an earth clamp at the meter to which is connected the earth wire of a modern consumer unit. This is the only true earth in the house because the water pipe is plastic and the iron gas pipe is, like the power cable, encased in bitumen. Would anybody like to hazard a guess as to what kind of supply this is?

This "clamp at the meter" - do you mean it's clamped around the incoming cable? If so, like my 1937 installation, it's TN-S (Earth is provided seperately from Neutral, using the cable-armour/sheath). A photo would make it easier to decide for sure, though.

Cheers,

Howard

 

[felix]

Thanks guys. Yes, the clamp is around the end of the cable, a point I see I didn't make clear. I was hoping it would be TN-S. I have only one service fuse marked 60 amps in the live side. Sorry about the lack of a photo but I haven't worked out how to post them yet.

Surely the change in L-E voltage under a known load current will give only the combined resistances of live and earth feeds. It would be nice to know the resistance of the earth alone. I remember looking for those free volts many years ago but found mere millivolts. If this is still the case, any substantially larger voltage appearing here with a known L-E load should give a reasonably accurate answer; unless of course you know different ---

PS: When I said "modern consumer unit" I meant modern in the sixties! No doubt other old hands will recognize this syndrome.

 

[mapj1]

Surely the change in L-E voltage under a known load current will give only the combined resistances of live and earth feeds. It would be nice to know the resistance of the earth alone. I remember looking for those free volts many years ago but found mere millivolts. If this is still the case, any substantially larger voltage appearing here with a known L-E load should give a reasonably accurate answer; unless of course you know different ---

Er yes you are quite right
What is measured in this way is always R1 PLUS R2.
for fault current this is the correct one to be considering, and for TT, where the spike impedance is high, its normally good enough on its own anyway.
If you really want earth impedance on its own it is not possible directly, the best you can do is if the L-N impedance can be assumed symetric, so half of it is L- only, and then an approximation for E on its own is possible, by measuring L-N loop, and then L-E loop. Or measure the drop in L-E with a load L-N, and then take that off the L-E drop measurement. Your last suggestion is not wrong, and may be a fast way of getting E on its own, if nothing else (other peoples L-N loads) is changing on the L-N loop. Probably OK for measuring late at night.
I think I'm making it worse, not better, so I'll stop now.
regards M.

 

[mikely]

But for a TT system why can't one earth the Neutral at the input of the installation like the diagram in http://www.iee.org/Publish/WireRegs/PME_ESQC.pdf
for at PME system?

Doing this would convert the system to TN-C because the installation earth is then connected to neutral and earth fault currents will flow via the neutral conductor. This is when a broken neutral is dangerous because all earthed metalwork is then connected to the neutral side of any loads and will rise to a potential only limited by the earth electrode resistances which are relatively high.

 

[felix]

To Mapj1:

I think I'm making it worse, not better, so I'll stop now.

Not at all. It all made sense. I wasn't sure about L-N symmetry but, now that I think about it, the dominant resistance will be in my own connection out to the street. I can even see the substation from the bedroom window which is surely a good thing in itself!

To Mikely:

I agree in part about that hybrid TT / TN-C being worse than straight TT. Depending upon the load at the time, a broken neutral can pull all the earth wiring up to dangerous levels relative to true earth. No fuses will go and I don't see that an RCD is any help. The pure TT system is safe in this condition because, capacitive couple aside, the earth wiring remains at earth potential where it belongs. The weakness of TT appears to lie in its relatively high earth wire to true earth resistance. A live to earth fault might not draw enough current to pop a large fuse before somebody gets fried. An RCD gets round this problem but only if the faulty circuit is protected by one.

Now for the million pound question: which is more likely in practice, the broken neutral or the non-RCD protected live to earth fault? That should keep this thread running to a few more pages ---

 

[Lectrician]

Take a look at the drawings in the last post of the For Reference Section.

 

[ban-all-sheds]

An RCD gets round this problem but only if the faulty circuit is protected by one.

That's why with TT supplies you are required to have one.

 

[felix]

Nice one Lectrician. A cracking set of drawings - and a reminder to muggins here to look in the reference section more often!

 

[Big_Spark]

WORD OF CAUTION TO ALL DIY'ERS

NEVER attempt to do an Earth Loop Test on the Supply to your property, it is extremely dangerous to attempt this if you are not 100% sure of the correct procedure and the dangers involved, it would also be illegal as you are not qualified.

The Supply cable does not fall under BS7671 or Part P, but the Electricity Supply Act, and this clearly states that all persons working on it if live must be hold recognised qualifications in electrical competences.

These cables do not have normal fuses backing them up in the street, they may have surge suppressing fuses fitting in the substation, but these will be rated such that any fault current and fault voltage you may be exposed to will likely be instantly fatal.

 

[mapj1]

Coming back to FWL engineer, don't we all effectively do this when we measure ELFI from the socket fed from the consumer unit? - the test current is still using the full supply earth loop for its path. Or did you mean that we should not be doing this test direct from the meter tails (I'd agree with the risk then, as the prospectice current in case of accident will be hundreds, possibly thousands of amps, at least briefly!)
- I'd certainly assume most testing is done on a final circuit, but that it is OK if the spur for the test is a very short one, made up for the test, thus would be acceptable.
It is certainly more safe than doing R1 +R2 measurements and only assuming the supply is OK - quite often it is not, particularly on older installations, where it is not unknown to find no earth at all.
Otherwise I have to disagree with you, this is no more dangerous than any other live working or test procedure, and competance, and a sensible safe approach, are required, not any particular qualification. (what qualifiction had you in mind?)
regards,
Mike

PS I'm not advocating that all people should do everything, but that all people should have enough information to decide on balance what they can, and cannot, safely do for themselves.

 

[Big_Spark]

Map...I said "Never attempt to do an Earth Loop Test on the SUPPLY to your property"

That is NOT a circuit within the property. Earth Looping in the installation has inbuilt safeguards, you still need to know what your doing as there are dangers you must be aware of, such as passing up to a 25A current through an installation!!

However i was specifically targeting the SUPPLY into the property as a few comments in earlier posts seemed to be heading in that direction and I wanted to avoid a situation whereby someone thought it safe to do, as it is not, regardless of the legal aspect of it.

 

[HDRW]

Map...I said "Never attempt to do an Earth Loop Test on the SUPPLY to your property"

Would this include connecting to the earth-clamp on the incoming cable, and the phase and neutral inside the CU? (That's where I'd do it if I was trying to find the basic loop impedance, for comparison with results from circuits in the house)

If this is a no-no, how do you know if a high impedance is due to something in the house as opposed to in the supply?

Cheers,

Howard

 

[Big_Spark]

Howard, If you want to do an ELIT on a circuit in the property and are 100% sure you know what your doing, then go for it, BUT I would strongly urge you NOT to do this in the supply side of your Consumer unit, this is potentially exceptionally dangerous.

I have read reports of DIY'ers being killed doing this.

 

[HDRW]

Howard, If you want to do an ELIT on a circuit in the property and are 100% sure you know what your doing, then go for it, BUT I would strongly urge you NOT to do this in the supply side of your Consumer unit, this is potentially exceptionally dangerous.

Thanks for your concern - I'm never happy to have the lid off my (old) CU because it exposes the supply side live terminals very prominently! (It's the old Wylex fusebox type with the wooden frame at the back). I shall heed your advice and leave it alone. So, how does an ELI of 0.38 ohms at a socket sound? (The house is TN-S)

I have read reports of DIY'ers being killed doing this.

Yes, I can well imagine. I personally know of three people who didn't survive a mains shock - a 14-year-old kid from the local sweet shop who was changing the lightbulb above his bed, the 17-year-old brother of a friend who was "messing about with a light" (I don't know any more details - not the sort of thing you ask), and a colleague who was standing in a garden pond, cutting the lining to shape, and went through the cable supplying the pump. The last one was only a couple of years ago, the others some time in the 60's. I've had a few mains shocks myself, the first one when I was about 7, from a frayed lead, and I try to avoid them in all circumstances!

Cheers,

Howard

 

[Big_Spark]

Howard, I would seriously look into getting your consumer upgraded. These old wooden frame types are rather hazardous and certainly implies that there would be other "nasties" waiting in the walls to bite you, or more readily your pocket.

Personally I would suggest calling ina Spark to do a FULL PIR on the installation. Find someone others have used and can recommend, don't necessarilly pick one out of the Yellow Pages. Talk to neighbours and friends and see if they have had good experiences with any.

The 0.38 Ohms for a socket is acceptable, not great, but certainly well within expected limits and implies that the installation has a good Earth from the supply.

I would lay odds that if the main board were replaced with a modern unit, and all the Main and supplementary bonding brought up to BS7671 standards, that 0.38 would drop to around 0.22.

I understand that cost may be a major factor in getting a porofessional in to do the job, but there is also the phased approach to this. Get the PIR done, then look at assessing the most critical parts that need attention and prioritise them

High Priority > Medium priority > Low priority

Examples for what I already know about your installation.

High Priority:

Replace Main Consumer Unit.
Upgrade, to BS7671 standards, all Main and Supplementary Bonding.
Rewire any circuits that do not meet absolute minimum standards for safety (Not necessarily BS7671 compliant, but are a serious risk of causing a shock or fire due to damaged cable etc)

Medium Priority:

Rewire all existing circuits and replace accessories.

Low Priority

Exterior Lighting (unless for absolute safety)

Now obviously the above is only a Guide, I do not know enough about your installation to say anything with any kind of merit except for the items I list in High priority.

Get in a spark, and listen to the advice, don't be afraid to ask questions, don't be afraid to say that you want to phase the work for economic reasons. Any decent tradesman would rather you be up front and only take on works you can afford to pay for at the time, it beats not being paid at all with all the problems and ill feeling it creates.