Is there any point testing earth loop impedance

[hoover]

In a tt system? The earth spike is Reading greater than 50 ohms so clearly the mcbs won't make their rated disconnection times. Waste of time?

 

[pevvers]

Not really, as you will still be showing that you've got an earth all through the circuit...

 

[nickblake]

incorrect an MCB is an overload device and will operate you can have a maximum value of 200 ohms with an earth spike and the installation has to be protected by an RCD so any earth leakage above 30 ma will cause the RCD to trip ,

 

[pevvers]

incorrect an MCB is an overload device and will operate

Not in the event of an earth fault in a TT system it won't, not by any stretch of the imagination.

Hence the reason to use RCDs for earth fault protection.

 

[nickblake]

Opps my error was thinking of overload not earth fault ,silly me must be getting late

 

[davelerave]

In a tt system? The earth spike is Reading greater than 50 ohms so clearly the mcbs won't make their rated disconnection times. Waste of time?

not very familiar with tt systems but obviously the loop-impedance is way too high for any automatic disconnection of earth faults by mcbs

so rcds will be required

 

[hoover]

Not really, as you will still be showing that you've got an earth all through the circuit...

But the cpc would be already confirmed when doing r1+r2?

 

[hoover]

not very familiar with tt systems but obviously the loop-impedance is way too high for any automatic disconnection of earth faults by mcbs

so rcds will be required

Of course.

 

[Des 56]

In a TT, short circuit protection is by the mcbs
They cant fullfill disconnection times with the higher rod reading for earth faults ,so an rcd does the job
The loop impedence test passes a high current through the earth path to determine a sound low enough reading to operate that rcd,without the test,we cant be sure of the reading or the earth integrity

We could forget the loop impedence test and measure the rod the proper way though

 

[Fox]

I measure mine with a ruler, although a retractable tape measure will generally suffice

 

[D Skelton]

how do we measure the earth rod and which method?

The full method is to use a three terminal earth rod tester with two temporary spikes, a current spike and a potential spike. You place the current spike at least ten times the length of the rod away (so for an 8ft coupled rod that's roughly 30m away) and take three measurements, one with the potential spike halfway between the rod and the current spike, another 10% of the distance away from the current spike towards the rod and then a final measurement 10% of the distance away from the rod towards the current spike. The tester then gives you an average value of the three readings taken.

Most of us however will just use our MFT to obtain an Ra value using the loop setting.

 

[Stroppy]

When using the spike method on an existing installation, how are you meant to know how long the rod is without pulling it out of the ground and probably ruining what could have been a good reading?

 

[Silly Sausage]

When using the spike method on an existing installation, how are you meant to know how long the rod is without pulling it out of the ground and probably ruining what could have been a good reading?

Have a look at the Installation Certificate! :smilielol5:

I've always wondered that myself!

 

[D Skelton]

Yes, as Archy said, look at the install cert, in larger installations the information will definitely be there.

If it's a smaller install and no cert then if it's a twig then it's a four footer, if it's a 5/8" then assume it's a coupled eight footer.

 

[Engineer54]

I wouldn't ever go shorter than 30m for the current spike, and normally it would be 50m for small commercial/domestic installations where possible!!
You can also use a dedicated Earth Tester in place of a loop tester at the service cut -out MET, using two leads!!