TT earth results

[brightspark26]

Hi I've been testing a garage installation and the house is quite remote and has a tt supply. The supply to the garage is swa supplied via a 32a mcb on a 30mA rcd main switch. The earth spike resistance is 21ohms. The earth fault loop impediance at the main switch is 22.1ohms. At the garage it is 22.1ohms.
The rcd is tripping within the parameters but I feel the results are way to high but I've been told its just how far away from the transformer the house is. Anyone got any suggestions

 

[Leesparkykent]

Hi I've been testing a garage installation and the house is quite remote and has a tt supply. The supply to the garage is swa supplied via a 32a mcb on a 30mA rcd main switch. The earth spike resistance is 21ohms. The earth fault loop impediance at the main switch is 22.1ohms. At the garage it is 22.1ohms.
The rcd is tripping within the parameters but I feel the results are way to high but I've been told its just how far away from the transformer the house is. Anyone got any suggestions

Why do you feel the test results are way to high?

 

[davesparks]

Is this a new installation of the earth electrode or an existing installation?
What work have you done?
What are the previous test results if the installation is existing?
What makes you say that the results are way too high?

Distance from the substation won't have any impact on the Ra, but will have a minute effect on Zs.

An Ra is acceptable if it is stable at that value. In theory any value could be acceptable as long as it is stable and meets the Zs requirements for the RCD.
But you should always get the Ra as low as possible.

 

[brightspark26]

Existing House we re-used the garage supply (garage got knocked down and a new one built) we then installed a 4 way consumer unit with sockets, lights, roller door etc in the garage
existing house db with rcd protection and 2 earth spikes with pit cover.

 

[davesparks]

Then it sounds like someone who actually knows what they are doing may have installed the rods. The bodgers generally only bother with one rod

 

[Leesparkykent]

I don't see what the problem is....

 

[brightspark26]

It was the company I work for that done it previously a few years ago.

 

[brightspark26]

Domestic tt installations are not something I come across a lot. Most tt systems I do are on building site generators were the earth spike result can be 100ohms but the zs at the db of the generator is 0.02 ohms etc due to the generator

 

[davesparks]

Domestic tt installations are not something I come across a lot. Most tt systems I do are on building site generators were the earth spike result can be 100ohms but the zs at the db of the generator is 0.02 ohms etc due to the generator

Generator installations are not TT and 100ohms is not acceptable for a generator installation

 

[brightspark26]

Generators are not my strong point !
Ive always been told to mark them as tt although the neutral/earth is linked inside
also been told cause it's got rcd protection the earth spike must be under 200 ohms !

 

[davesparks]

Generators are not my strong point !
Ive always been told to mark them as tt although the neutral/earth is linked inside
also been told cause it's got rcd protection the earth spike must be under 200 ohms !

Utter nonsense!
A generator with a N-E link referenced to earth is a TNS earthing system.
Ask whoever told you this to show you the regulation which says that the limit is 200ohms for a TT or TNS earth electrode!

 

[brightspark26]

Say the Main switch rcd on the generator is set to 100ma

 

[davesparks]

Say the Main switch rcd on the generator is set to 100ma

Ok, what's that got to do with it?

 

[brightspark26]

Maz ze should be 0.8 ohms ?
Whats the maximum valve allowed for the spike

 

[davesparks]

Maz ze should be 0.8 ohms ?
Whats the maximum valve allowed for the spike

That's for a DNO supply, not a generator! You will struggle to get that on a generator due to Zgen. But Ze has nothing to do with the earth electrode on a generator, it's not in the fault loop!

There is no maximum value set for the earth reference, it's pretty much outside the scope of bs7671 and more in the realms of various earthing standards.
20ohms is often suggested as this is the theoretical max for a TNS at a substation

 

[sparkymalarky]

BS7430 - Earthing for generator 20 ohms or less for Ra.

 

[brightspark26]

You will love this One. Years ago our supervisor made another tradesman get the value under 200 ohms on a site being supplied via a big generator. They couldn't get the value down so they ennded up getting old crash barriers buried to try and get under the 200 ohm value. So basicly the earth spike is just protecting the generator frame and needs no value recorded ?

 

[davesparks]

BS7430 - Earthing for generator 20 ohms or less for Ra.

Thanks

 

[davesparks]

You will love this One. Years ago our supervisor made another tradesman get the value under 200 ohms on a site being supplied via a big generator. They couldn't get the value down so they ennded up getting old crash barriers buried to try and get under the 200 ohm value. So basicly the earth spike is just protecting the generator frame and needs no value recorded ?

The earth spike isn't protecting anything. It is providing the earth reference for a TNS supply. It ties the neutral point of the generator to the potential of the earth.

 

[brightspark26]

How could we ever get it to 20 ohms when we can't get it to 200