# No Earth from Supply on front of terraced house

Before a big argument breaks out thats 21 ohms is the max resistance on the suppliers side of a tt sytem you have to add the 21 on to the installations earth electrode

that's what i thought, it's just the wording that threw me.. "negligable impedence..."

I've never done a TT install, but if I do, I'm using 16mm for the MET to the earth rod and sod the lot of ya

sparks123 you got there just before me you must have been typing at the same time as me.

Just out of curiosity how did this forum subject start????? I forgot as it went on a strange tangent.

so the I value, is the current required to "blow the fuse" in the required time? aquired from the fuse / breaker graphs in the back of the regs?

so for a BS88 100A in 0.2s it's 1150A?

Just out of curiosity how did this forum subject start????? I forgot as it went on a strange tangent.

some guy in a terraced house wants to put an earth rod in the middle of his lounge because the DNO didn't supply him with PME...

But with a TT supply the impedence is too high to ever blow a fuse like that, hence RCD protecting every circuit.

So if you take the current which can flow in a fault, i.e. 230/Ze for say a fault path of 50 ohms (which is perfectly allowable), a current of 4.6A will flow.
A 30mA RCD will switch off quickly - in less than 40mS.

S = (sqrt I²t) / K

S = (sqrt (4.6² x 0.04)) / 143

S = 0.006mm²

You lot are way behind with the fault current calculation! Me and aptsys broached this a few pages ago.

Question: What might be a best case likely current to earth on a TT supply caused by a dead short phase to earth?

Let's say best case scenario for Ze: 22 ohms. Highest voltage 252.

252/22 = 11.45A

I think it was flameport that correctly identified the "OR" word.

Yes, Scot & Col, you can use the "rule of thumb" and use 16/10mm², or you can use the adiabatic to calculate the sizes from the fault current. Which is what aptsys & myself were getting at.

It just proves you don't have to follow convention when designing installations.

Sure, I can see your point: there is no harm in using the standard conductor csa's on a TT install. However, you don't have to: you can install a 2.5mm² earthing conductor & 6mm² PE bonding conductors on a TT install and be perfectly compliant.

It reminds me of an excercise in class on my C&G course back in the 80's.

We had to calculate the minimum csa of T&E for a heater circuit of Xm. We all surprised ourselves by discovering that we could safely do it in 1mm², even though convention uses 2.5mm².

There is often more than one way to skin a cat!

BTW, I have discovered a footnote to table 41.1 with regard to TT systems:

Where, in a TT system, disconnection is achieved by an overcurrent protective device and PE bonding is connected to all ex-c-p's within the installn in accordance with 411.3.1.2, the max disc. times applicable to TN system may be used. In other words, you can scrap the 0.2s and use 0.4s instead.

hell of a small cable but in theory ok however in practice would you work it all out or just go by the general rules and use 16mm?

As I said,

Sure, I can see your point: there is no harm in using the standard conductor csa's on a TT install. However, you don't have to: you can install a 2.5mm² earthing conductor & 6mm² PE bonding conductors on a TT install and be perfectly compliant.

i am getting to think this discution has run its course and everyone is both agreeing and disagreeing with each other and it comes down to either keeping it simple and going with the common rules or using adiobatic which may give a smaller cable but in most cases in reality a 16mm would be put in to keep testers happy

Wasn't it the case that I said you could use 2.5mm² for an earthing conductor on a TT supply & you insisted it was 16?

Does this mean you will not be issuing code 1's if you come across a TT supply with a 2.5mm² now then?

If I come across an installation with seemingly undersized conductors, I do a Ze and work out the fault current, then a quick dabble with the calc. will tell me whether they are compliant or not.

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