MAX Ze value for TT?

[Toby Kuhnert]

Hi,
I'm in my second year at college, but I'm a bit unclear on what the max Ze should be for a TT system. I know that the regs say 200ohms, and my tutor said that it would be acceptable if you found an earth rod which gave a reading of 200ohms. However several of my mates who have been qualified for years, tell me that they don't agree. They won't certify a TT install if it has a Ze of more than 20ohms?

What's everyones opinion on this?

Thanks in advance.

Toby.

 

[telectrix]

They won't certify a TT install if it has a Ze of more than 20ohms?

that's crap. unless you use 2 5/8" rods screwed together, you'll be very lucky to get under 100 ohms. this 20 ohms comes from a misunderstanding confusion over DNO earthing max. of 21 ohms. all TT systems must be backed up with 30mA RCD on all circuits so the max Zs for any circuit is 1667 ohms.

 

[ackbarthestar]

The earth path is made up of several sections

1/ The suppliers earth electrode , usually quoted as 21 ohms max.

2/ Resistivity of the soil, could be as high as 20k ohms

3/ the contact resistance between the installation electrode and the surrounding ground, this is where you come in as the sparks.

The maximum Ze allowed by the IET is 200 ohms, NICEIC = 100 ohms,

 

[ackbarthestar]

Sorry Tel I just wasn't fast enough

 

[telectrix]

i blame the zoo keeper.

 

[Toby Kuhnert]

Thanks tel, that sounds right. I had heard of a 20ohms max from somewhere, that's why I really wasn't sure. How come the NICEIC require 100ohms, when the regs state 200ohms? :S

 

[ackbarthestar]

So do I, especially when she's got a bag of carrots

 

[telectrix]

How come the NICEIC require 100ohms, when the regs state 200ohms?

because the niceic think that they're God. superior to the IET, so much so that they endorse a visual inspection of an installation , when 90% of sparks do proper test/inspections

 

[Toby Kuhnert]

Are there any other electrical bodies, like elecsa, who have their own set of "regs" then? or is it just the NICEIC?

 

[ackbarthestar]

How come the NICEIC require 100ohms, when the regs state 200ohms?

because the niceic think that they're God. superior to the IET, so much so that they endorse a visual inspection of an installation , when 90% of sparks do proper test/inspections

I can't understand why the IET have chosen 200 Ohms when its obvious why 100 Ohms was chosen by NICEIC

Zs = 50 / 0.5

Why 50V, lets say touch voltage
Why 500mA , Lets say maximum acceptable current rating of a domestic/commercial RCD providing the maximum earth fault path resistance for instantaneous disconnection under the 16th ed. BS7671

I suspect that the NICEIC will be updating their advice soon.....

 

[sjm]

Thanks tel, that sounds right. I had heard of a 20ohms max from somewhere, that's why I really wasn't sure. How come the NICEIC require 100ohms, when the regs state 200ohms? :S

The simple answer is they don't!!
I'm not sure where the 100 ohms NICEIC rumour came from but it's wrong.

 

[ackbarthestar]

The simple answer is they don't!!
I'm not sure where the 100 ohms NICEIC rumour came from but it's wrong.

You might like to read of the NICEIC tech manual and you will see it stated there.

As you well know its logical that to maintain a touch voltage of less than 50V under earth fault conditions when using a 500mA up front RCD you will need a resistance of less than 100 ohms

R <= 50 / 0.5 = 100 ohms

BS7671 now states a maximum front end RCD of 300mA, the one exception, at the moment is construction sites where 500mA is still recommended.

However, if you check manufacturers info you will probably find that there are RCDs (RCCBs) that are adjustable to 10A for large sites where there are combinations of RCDs in series providing discrimination at different level, such as caravan sites

 

[telectrix]

but what is the rating of a RCD in a zoo, is it 200mA? ( Z00mA)

 

[sjm]

Which tech manual?

In the Inspection, testing and certification guide I have seen that they point to "BS7430: Code of practice for earthing which suggests that a value of earth electrode resistance exceeding 100 ohms MAY be unstable."

The NICEIC themselves have never required a max value of 100 ohms for an earth rod, simples.

 

[Engineer54]

The truth of the matter is, that both 200 and 100 ohm Ra are pretty much useless values!! A far cry from the old 10 ohm Ra of yesteryear!!, Even the States call for a max of 20 ohm's Ra and an additional rod (all 10 footers by the way, ...lol ll ) if the first rod don't reach 20 ohms!!