Bringing down Ze?

[Ross1]

Hi,

Im getting a Ze reading of 580 ohms on a TT installation, the existing earth to rod is either a 2.5 or a 4mm and im not sure of the route.

My question is if I run a 10mm earth (approx 35m) how much will it bring down the Ze?

 

[JLD]

Think it was in hope the bigger the CSA of the main earthing conductor, the lower the Ra would be.

I agree the larger the conductor CSA is the lower the resistance. My question that I perhaps could have worded better is why 10mm2 and not 16mm2 in accordance with BS7671 reg 542.3.1 and Table 54.1?

 

[Engineer54]

Why?? ....Because most of the time in the UK, you bung in short thin rods that are not going to produce low Ra levels.... hence low fault levels, hence low conductor sizes. Everything is relevant!! lol!!!


And if your thinking that by installing anything bigger than what's already there, will decrease your rod resistance, ...think again!!:angel_smile:

 

[DurhamSparky]

My question that I perhaps could have worded better is why 10mm2 and not 16mm2 in accordance with BS7671 reg 542.3.1 and Table 54.1?

i think you shoudl re read your regs book matey!! TT installs dont have to be 16mm Earth..! TN installs DO!

 

[JLD]

Why?? ....Because most of the time in the UK, you bung in short thin rods that are not going to produce low Ra levels.... hence low fault levels, hence low conductor sizes. Everything is relevant!! lol!!!


And if your thinking that by installing anything bigger than what's already there, will increase your rod resistance, ...think again!!:angel_smile:

Perhaps we are talking at cross purposes as the original thread was a high reading which the writer wanted to know how to reduce not increase. Why on earth would you want to increase your earth rod resistance if you feel it is too high???? In the original post the writer stated they were going to install a 10mm2 cable to reduce the value of earth fault loop impedance and my question was why only 10mm2 and not 16mm2 as per BS7671. The purpose of a low fault path resistance is to enable sufficient current to flow during fault to enable disconnection within the required time. :6:

 

[tony mc]

I would try the PME route if at all possible.

If not can you not install a rod (rods) at a new position trying to get the Ra down.

As IQ has said the cable resistance is not he issue as 2.5 would only be around 1/4 of a ohm for 35 meters!

 

[Engineer54]

Perhaps we are talking at cross purposes as the original thread was a high reading which the writer wanted to know how to reduce not increase. Why on earth would you want to increase your earth rod resistance if you feel it is too high???? In the original post the writer stated they were going to install a 10mm2 cable to reduce the value of earth fault loop impedance and my question was why only 10mm2 and not 16mm2 as per BS7671. The purpose of a low fault path resistance is to enable sufficient current to flow during fault to enable disconnection within the required time. :6:

''will increase your rod resistance,''

I think that was an obvious typo error on my part, (now corrected) don't you think, ...considering everything else stated in my posts on this thread...

 

[GLENNSPARK]

Depth..depth..depth all the time mate....resite a new (extendable) rod and keep hittin em in until the Ra comes down (if pme isnt available) but thats already been covered here......what a stupid place to build a conservatory....lol........

 

[Octopus]

How about a new stake outside in the garden, away from the house? Sure it'll add a few more metres but having it in "better" ground has to be considered.

 

[Des 56]

having it in "better" ground has to be considered.

Encased in concrete may be better than soil

It puzzles me how these earth stakes end up indoors,surely the building damp membrane is shot if its spiked, in most cases

 

[GLENNSPARK]

How about a new stake outside in the garden, away from the house? Sure it'll add a few more metres but having it in "better" ground has to be considered.

Agreed murdoch...a few more meters is nowt to get a better site......

 

[Ross1]

Thanks for all the input fellas, now the cu is by the front door and the rod is at the back of house as said in conservatory, hence the 35m run.
Next door neighbors rod I noticed isjust outside the front door so I could try knocking a rod in there ( no need for a 35m run then)

Any advice on installing an earth rod as I've never done it before? Is it a sledge hammer job?

 

[IQ Electrical]

Thanks for all the input fellas, now the cu is by the front door and the rod is at the back of house as said in conservatory, hence the 35m run.
Next door neighbors rod I noticed isjust outside the front door so I could try knocking a rod in there ( no need for a 35m run then)

Any advice on installing an earth rod as I've never done it before? Is it a sledge hammer job?

As my learned friend E54 will advise, don't just drive a rod right next to the building, there's a fair chance you'll be hitting builders rubble etc. and will really struggle with Ra values.

You can couple rods together to get more depth, use a lump hammer rather than a sledge hammer, you'll just end up damaging the rod, especially some of the pathetic 9mm rods that seem popular!

 

[Ross1]

As my learned friend E54 will advise, don't just drive a rod right next to the building, there's a fair chance you'll be hitting builders rubble etc. and will really struggle with Ra values.

You can couple rods together to get more depth, use a lump hammer rather than a sledge hammer, you'll just end up damaging the rod, especially some of the pathetic 9mm rods that seem popular!

How long are these rods generally and what depth would be a good start mate?

 

[Des 56]

I noticed some of the lads on this site suggesting an item that fits in a drill that is used for impacting the rod into the ground
I have always used brute force with the trusted lump hammer,so cant comment more on them.but maybe someone else can

 

[topquark]

They're 1200mm or 4ft in old money.