TT vs TN-C-S

[ban-all-sheds]

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.

Should Ze be measured literally at the incoming service position, or can it not be done between the CU incomer and the MET? Mind you - the current available to you after the service fuse is hardly less lethal than that before it.

I'm curious to know why you'd want to measure Ze, though. If you get a high Zs, and R1+R2 checks out OK, how many other places are there to look?

 

[theshogun]

The value of ze is a requirement on a certificate,(Nature of supply).
It is also needed with respect to

zs=ze+(r1+r2)
zs is mormally lower because of parallel earth paths if it is very high it could mean the bonding of the installation is not up to scratch.

However this could be obtained from the supplier ,without the need to measure it .

 

[Damocles]

HDRW, you seem to have had extraordinary bad luck. If everyone in the country knew three people who had been electrocuted there would hardly be anyone left alive. I had the definite impression that the annual electrocution rate was tiny.

 

[Big_Spark]

Dam...I think it is safe to say don't offer to help him out with Electrical DIY...we would likely need a new Mod!!

 

[Big_Spark]

B-A-S, generally you would not measure it, as Shogun has said, you would calculate it or get it from the REC.

I have measured it on several occasions.

Regarding the fuse comment, I take the point your making, but the supply fuse will limi any fault current ot 100A..still ouch, but without that fuce you could be exposed to a potential fault currrent of several tens of thousands of volts and several thousand amps.

 

[Damocles]

Careful! muddling up your volts and amps is dangerous. Is that an induced emf of 10,000V or does the transformer melt and short out, exposing you to the high voltage side?

Sorry, that was cheap. Its getting late and I should have gone to bed long since. No screaming tonight to excuse my staying up. Even the dogs stopped whining.

 

[HDRW]

FWL: Thanks for your concern - I am taking your advice seriously, although not necessarily planning to take all of it as it stands...

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.

This is in hand, because apart from anything else the CU hasn't got enough ways for my future plans. I do know there is some VIR cable - two short runs from the CU to junction boxes for lights and immersion heater, and the immersion heater is VIR all the way. Of course if this gets accidentally damaged, I can replace it without breaking Part P, can't I? I know there is disused VIR under the floorboards (I found the cut-off ends) but I don't know if there's any still in use beyond what I said above, but I can find this out myself.

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.

Talk to the neighbours? This is Hertfordshire - we don't do that!

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.

Well that result is what I got from a socket that I suspected had a dodgy earth (has very little slack in the cable so difficult to see if the earth is connected properly) so I'm rather glad it's acceptable!

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.

That's interesting - is that "the usual figure"?

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

The main difficulty is that my house is full! Just getting under the floorboards (in fact getting into some of the rooms) could be the work of several months, involving a skip... can you tell I live alone?

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)

As I said, replacing the CU (and getting an isolator in front of it) is the first thing I'm planning. I know the earthing is nowhere near the current standard because the link from the earthing block to the CU is (a) black and (b) looks like 2.5mm!

Medium Priority:

Rewire all existing circuits and replace accessories.

The switches and sockets themselves are OK, but backboxes are an issue - there aren't many! Lightswitches tend to be over holes in the plaster with woodscrews holding them in place. I wonder if this is typical for a 1960s rewire?

Low Priority

Exterior Lighting (unless for absolute safety)

That's easy - there isn't any!

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.

I wish all tradesmen were as reasonable as that. My experience has been that an awful lot of them try to "build the job" and treat punters as idiots to be milked - I know what you said about asking for recommendations but I usually get the reply "Well, I wouldn't recommend the last bloke we had in"!
Thanks very much for your advice, I will be taking it into account, even if I don't follow it to the letter!

Cheers,

Howard

 

[Big_Spark]

Careful! muddling up your volts and amps is dangerous. Is that an induced emf of 10,000V or does the transformer melt and short out, exposing you to the high voltage side?

Sorry, that was cheap. Its getting late and I should have gone to bed long since. No screaming tonight to excuse my staying up. Even the dogs stopped whining.

I appreciate your being facecious, but to explain...

A direct short from Phase to Earth will create a fault voltage which is a product of the characteristics of the circuit, and one of the factors governing this is the protective device. This voltage can range from a few thousand volts to over 100,000 volts. Domestic single phase circuits rarely have a fault voltage in excess of 16kV, however the supply to the property can have a potential fault voltage in excess of 100kV, depending on the type of supply.

Also there is a Fault current, measured in kA, this is also a product of the characteristics of the circuit in the same way as the Voltage is, and the two are directly related together. In domestic single phase circuit the fault currents rarely rise above a few kA, this is why MCB's and RCBO's are rated at a breaking current of usually 16kA. Again the supply is different, and the potential fault current on it can easily exceed 20kA.

Now both the voltage and the current are momentary, lasting hundreths of a second, however should such voltages/currents pass through the human body then they will prove fatal.

 

[HDRW]

HDRW, you seem to have had extraordinary bad luck. If everyone in the country knew three people who had been electrocuted there would hardly be anyone left alive. I had the definite impression that the annual electrocution rate was tiny.

Well some would say that it's people I know who've had the bad luck (and I didn't even mention my best friend at school who died in a motorcycle accident when I was 16). As for the annual rate, two of those I mentioned were forty-odd years ago, and one I didn't know, but I know his sister now. Only my former colleague was recent, and I can't help thinking that if he'd been more careful and/or had an RCD in the circuit it wouldn't have happened. I always bear in mind that he wasn't an idiot, and probably just didn't think through what he was doing - I try to make sure I've covered all the safety aspects when I'm doing anything potentially dangerous.

I recently did a "safety update" for a friend (who'd hate to be called an elderly lady despite being in her seventies!) because I was worried that there was potential danger there.

I installed an RCD "plug" on her lawnmower, replaced an indoor-type lightswitch in her greenhouse with an IP56 one, and changed an indoor-type socket in her garage for an RCD-equipped one (and corrected the fact that the previous one had L & N reversed!).

Before anyone picks up on the greenhouse - it's a lean-to type built against the garage wall, and the switch is on the wall itself.

If anyone is going to say that I've breached Part P, then sorry, but I consider a wrongly-wired socket much more dangerous than what I've left. I double-checked everything I did, and tested it afterwards. I do know what I'm doing at this level, even if I don't have the paperwork to prove it!

Cheers,

Howard

 

[ban-all-sheds]

I appreciate your being facecious, but to explain...

I'm looking forward to this.

A direct short from Phase to Earth will create a fault voltage which is a product of the characteristics of the circuit, and one of the factors governing this is the protective device. This voltage can range from a few thousand volts to over 100,000 volts. Domestic single phase circuits rarely have a fault voltage in excess of 16kV, however the supply to the property can have a potential fault voltage in excess of 100kV, depending on the type of supply.

So the supply to the property has a PD of 230V RMS between live and neutral, and (more or less) live and earth.

And in a fault this rises to tens or hundreds of kV.

How does that work?

Now both the voltage and the current are momentary, lasting hundreths of a second, however should such voltages/currents pass through the human body then they will prove fatal.

Voltage doesn't pass through anything.

 

[ban-all-sheds]

The value of ze is a requirement on a certificate,(Nature of supply).
It is also needed with respect to

zs=ze+(r1+r2)
zs is mormally lower because of parallel earth paths if it is very high it could mean the bonding of the installation is not up to scratch.

Yup - I know all that

However this could be obtained from the supplier ,without the need to measure it .

Yup - Nominal voltage & frequency by enquiry, Ze by enquiry or measurement

 

[ban-all-sheds]

B-A-S, generally you would not measure it, as Shogun has said, you would calculate it or get it from the REC.

Yes I know - hence my question..

I have measured it on several occasions.

Was that because the DNO wouldn't tell you, or because you had reason to believe their value was wrong?

And between which points did you measure it?

 

[mapj1]

And if you KNOW that Southern electric will say "a third of an ohm", for all houses on that branch regardless of the particular property - even in one case one that turned out to be TT of sorts, albeit without the incoming RCD, you might like to measure it, just for a warm feeling. Personally, as my tester ends in a 13A plug, I have extension lead style socket, that I just hook to the bars in the CU and a conventient fuse or circuit breaker, making a really short spur, for the duration of the test. Also handy for emergancy supplies when all else is disconnected.
regards M.

 

[Damocles]

i am not entirely sure about the list of priorities. It might be I would put attending to the rubber wiring at the head of the list. If it is still in good condition then fair enough. Beware it might fall to pieces if in bad condition and you start moving it. Most likely at the cable ends where it heats most and air can get at it.

This house has quite excellent wooden boxes in the plaster for the light switches. I suspect they pre-date the current cabling. No chance of a shock off them. Did they use that fireproof wood the same as on the Kings Cross underground elevator?