cure for poor Ze on TT

[markthebuilder]

1)Pull earth rod out using your teeth for maximum impact
2)wack rod over apprentices head TWICE
3) Then straighten then out any major bends drive rod in to more new ground
4) reconnect and test
hey presto 256 to 129 ohms in 4 easy steps

 

[markthebuilder]

spark 123
Just seen your response to another thread What grounds do NECEIC ask for a different standard for Ze on TT other than it being safer
Guess I will be putting an additional spike in after all

 

[Taylortwocities]

A word of warning. Just banging in another earth stake next to the existing one will not increase the earth fault impedance by much. Earth electrodes effect each other and if they are too close together then you're wasting your time.

Better to get a rod that you can screw another on to (making it twice the length) or a thicker rod (bigger surface area).

Its a black art this malarky and I cant find the ref offhand but I found this though:

"Where more than one rod is necessary, then a number of rods may be installed at a distance of approximately, but not less than, 3 metres from each other. The resistance of N rods at 3 metre spacing, connected in parallel, is approximately 1.5 x R/N where R is the resistance in ohms of one rod." This means that if you have one rod and it gives u an impedance of 500ohms and then you bang in another rod 3 metres away then the resulting impedance will be in the region of 375 ohms (not 250!!)

Well outside the scope of a DIY forum but maybe someone (supersparky??) knows the full facts off the top of his head?

TTC

 

[Spark123]

spark 123
Just seen your response to another thread What grounds do NECEIC ask for a different standard for Ze on TT other than it being safer
Guess I will be putting an additional spike in after all

The NICEIC seem to insist on this figure quoting from P215 of the NICEIC Inspection, Testing & Certification handbook: "BS7430 Code of Practice for Earthing which suggests a value of earth electrode resistance exceeding 100 ohms may be unstable......"
The IEE on the other hand say in Guidance Note 5 (p40) "for reliability over time in normal circumstances the earth electrode resistance should not exceed 200 ohms whatever the rated residual current device rating."

 

[Maxxy]

Better to get a rod that you can screw another on to (making it twice the length) or a thicker rod (bigger surface area).
TTC

Providing they dont have big chuncky connectors that are wider than the actual rod, thus making a big gap around the top rod

 

[Taylortwocities]

two 14mm rods joined together with one of these, maybe?
http://www.tlc-direct.co.uk/Products/TLER58CUP.html

TTC

 

[plugwash]

A word of warning. Just banging in another earth stake next to the existing one will not increase the earth fault impedance by much. Earth electrodes effect each other and if they are too close together then you're wasting your time.

Better to get a rod that you can screw another on to (making it twice the length) or a thicker rod (bigger surface area).

Its a black art this malarky and I cant find the ref offhand but I found this though:

"Where more than one rod is necessary, then a number of rods may be installed at a distance of approximately, but not less than, 3 metres from each other. The resistance of N rods at 3 metre spacing, connected in parallel, is approximately 1.5 x R/N where R is the resistance in ohms of one rod." This means that if you have one rod and it gives u an impedance of 500ohms and then you bang in another rod 3 metres away then the resulting impedance will be in the region of 375 ohms (not 250!!)

the maths of earth electrode resistance is pretty horrid for all but the simplest cases, in most cases i'd imagine its done by just adding more metal until its low enough following rules of thumb like this to avoid wasting rods.

btw i remeber reading somewhere that if your building has a reinforced concrete foundation then provided the rebars of that foundation are well connected to each other they make a surprisingly good earth electrode.

 

[JohnD]

Some sites use an "earth pit" being a trench where copper strip is laid in a lattice then filled in. It must give a good wide contact.

 

[Taylortwocities]

Yes, it works well BUT (there is always one!) must never be used in agricultural areas where livestock can stray into the area of the earth mat.

They don't like it up 'em Mr. Mannering!!!


TTC

 

[securespark]

But the best cure for shoite Ze on TT is to get the DNO to PME it, then you don't have to worry about TD RCD. Round us, it costs £99.

 

[JohnD]

Better buy some 16mm G&Y then.

 

[ricicle]

As far as I am aware PME is not always available on all TT systems.The DNO's neutrals have to be earthed every so often on their poles

 

[Taylortwocities]

True ricicle. Request to DNO will be granted if poles already rodded.

TTC

 

[markthebuilder]

Thanks for the info peoples Hind site is a wonderful thing but the job has proved an excellent training excersise I think a phone call to the DNO will be first stage of any TT job in future