TT Max Ze

[EddieB]

Guys, what is the maximum Ze allowed using a TT system

 

[malcolmsanford]

Are you working to the Regs, the BS 7430, the OSG or to something more sensible?

 

[telectrix]

200ohms is considered the max. before becoming unstable. niceic prefer 100, i prefer as low as you can get it.

 

[EddieB]

If the reading is over 200 ohms how would you improve it? the existing earth rod is reasonably new

 

[topquark]

If the reading is over 200 ohms how would you improve it? the existing earth rod is reasonably new

Either more rods, or if you're lucky and the rod is one of the threaded ones then attach another to the top and drive it deeper.

 

[Engineer54]

Eddie,

Is this for your own house, or for a job your doing??

 

[silver surfer]

I thought the max Ze was 21 ohms on a TT ?

Can someone clarify please?

 

[malcolmsanford]

That is the maximum that the DNO have to ensure at their earth electrode is at the transformer.

 

[silver surfer]

How did they come to the figure of 21 ohms?

 

[telectrix]

picked it out of a hat.

 

[Octopus]

If the reading is over 200 ohms how would you improve it? the existing earth rod is reasonably new

I'd move the rod, maybe water it first.

 

[IQ Electrical]

If the reading is over 200 ohms how would you improve it? the existing earth rod is reasonably new

Remember that the maximum allowed value of Ra is 1667 Ohms assuming 30mA RCD protection.

The guides mentioned give figures of between 100 and 200 Ohms as a guide value to allow for soil drying and freezing, possibly pushing the value of Ra towards or even over the magical 1667 Ohms.

If you have a deeply driven rod with an Ra reading of say 400 Ohms, this will probably be almost immune to soil drying/freezing due to the depth and will therefore be far more effective than a shallow-driven rod with an Ra of 100 Ohms.

 

[telectrix]

can't go too deep though as the rod will melt when it penetrates the earth's molten core.

 

[IQ Electrical]

can't go too deep though as the rod will melt when it penetrates the earth's molten core.

I've never actually seen this happen and I've reached the core a few times, are you using cheap rods?

 

[malcolmsanford]

can't go too deep though as the rod will melt when it penetrates the earth's molten core.

Don't Tel as we seem to be struggling enough as it is ......................