Scariest Post/ Thread?

[securespark]

Have been flitting about reading some vintage stuff.

Think this has to be a nomination: http://www.diynot.com/diy/threads/very-high-earth-loop-impedence-zs.173111/

Perhaps we should set up an award each year for the scariest?

 

[ericmark]

Oh I see what you mean, it was 2009 however maybe he has re-trained now? Because at one point my job for 6 months was installing earth rods and measuring their impedance I am fully aware of how to install an earth rod, both from scratch and using the DNO supply as a reference point. But having read some PIR and EICR and seem the reading given it is clear other electricians don't seem to know what they are doing.

When I see a TT installation with a Ze of 10Ω or less it rings alarm bells. Not saying you can't get a Ze of 10Ω but in the main I find water and gas pipes were left connected when measured. The external loop impedance to my mind should be the electrically permitted earth connection and not any part which could be removed by non electrical personal. So should not include any connection to gas or water pipes, but could include connection to re-bar in the building foundations.

To measure the Ze below 50Ω really does need the two test probe method. To compare to the DNO supply we have to remember they are only required to have a 15Ω earth so with a reading of 200Ω the 15Ω is not really a problem, but to get a reading of 8Ω unless you know the DNO reading then the two electrode method is the only option. However although with the gas terminus I was working on one can get the distance required, with a domestic getting electrodes in the ground to measure the earth rod is often impossible.

As to if we should record the Ze as <XΩ where we use the DNO as a reference point is debatable.

However when I was at collage I have to admit I was not shown how to measure Ze until I did my C&G2391. By that time I had used the earth rod measuring kit as I had needed to install earth mats for galvanic protection, so came out of my time 1972 but it was 1980 before I had to measure earth mats. Do remember at that time collage qualifications were not required to be classed as an electrician.

So although clearly bad that some one who should know how to fit rods does not, however it is really a reflection on the scheme providers that they can accept people who clearly have not been trained.

The question must be, is there any point in the scheme providers if they don't test electricians to find their skill level? However no one has zero potential, even when I did the job for 6 months I sill did not have zero potential even if that was the standing joke.

 

[ban-all-sheds]

I expect some have alternating potential - varying by time of day and/or day of week.

It is an ancient Sparky,
And he worketh one of three.
The other days he stayeth in bed,
Too drunk to stand or see.​

 

[ericmark]

I was considering negative potential, but could not image the number maybe J?

Clearly we scared him off, not seen since 2009. But I remember when working on the Falklands the spares had not arrived, so no voltage regulators for the wagons. Could however frig the alternator to give a small charge so batteries took longer to go flat than simply charging over night while we waited.

Another electrician working with me I had worked out needed around 10Ω so said to him needs around 10Ω I've got a break down fit any old bulb holder and use a bulb of around 10Ω. Reply was how do I work it out? I said use ohms law, reply what is that?

I looked at him in complete amazement, thought he was having a joke, then realised he was serious he really didn't know. He was trained by the forces, it would seem mainly on the job, and he actually thought an electricians job was unplug one bit and plug in a new bit until it worked.

Thank god he only worked on 12 and 24 volt. I did not dare let him lose with generator sets. As to our 3.3kV distrubution system!

 

[PBC_1966]

One part of that reminds me of an argument I had with a supposedly "fully qualified" electrician elsewhere some years ago who seemed to think that adding an earth rod to a TN-C-S system converted it into some sort of semi-TT system and was dangerous.

 

[Spark123]

When using lamps as resistors I tend to find it a try and see scenario, a cold filament will give a different resistance to that of warm one and different again to a hot one.

 

[PBC_1966]

a cold filament will give a different resistance to that of warm one and different again to a hot one.

There are some thermistors which are basically just small lamps. They were used in the regulators of GPO/BT 700-type telephones, for example.

 

[Echo the husky]

Just whack a barretter in there!

 

[Jackrae]

Also used in radios as voltage "stabilisers", probably on he basis of current control; the more current that flowed, the hotter it got so the resistance increased to reduce current.

 

[Echo the husky]

That's how I know about them, I collect valve TVs and Radios!

 

[ericmark]

I do agree however it was the fact he did not know ohms law.

As to earth electrode with a TN-C-S it changes name to extraneous conductive part and so the statement you can't have an earth electrode with a TN-C-S system is correct, but not because you can't have a metal rod in the ground, it is because it's not called an earth electrode.

 

[ban-all-sheds]

Unless it's installed by the DNO.

So if you don't know who has installed it, is it earthed or bonded?

 

[Risteard]

I disagree. You most certainly can have an earth electrode with a TN-C-S system (and indeed most countries require it).

Although BS7671 does not require an earth electrode for a TN-C-S system, where one was installed I would not class it as anything other than an earth electrode.

 

[Spark123]

I don't think I'd be too happy with a smaller conductor to the electrode than one for a main protective conductor tho.

 

[PBC_1966]

I disagree. You most certainly can have an earth electrode with a TN-C-S system (and indeed most countries require it).

As here in the U.S. where TN-C-S is the norm and the NEC requires an earth electrode connected to the incoming neutral at every house.

If you shove a few feet of metal rod into the ground, it's going to act as an earth electrode whatever you want to call it. That was part of the issue I referred to earlier, when it was being argued that adding an electrode to a TN-C-S system was somehow dangerous. I asked for explanation of exactly how it differed from bonding a underground metallic water pipe entering the building, which even if coupled to the main with an insulating insert is still likely to be a far more effective electrode that a 4 ft. rod driven into the ground (and he was agreeing that for safety such a pipe needed to be bonded). His argument against that was that BS7671 "doesn't allow a water pipe to be used for earthing." Very true, of course, but nothing could get him to see that the pipe was still going to behave as an earth electrode anyway.