Converting to a TT system... possibly

[chris day]

1.25ohm external earth fault loop impedance on TN-S supply to building

Supply authority stated our only option is a TT conversion as they will not covert to PME.

What would be my best course of action when converting to a TT as the building is quite old and i do not want a 100ma main switch for diversity reasons and nuisance tripping, so Would it be suitable to convert to TT and have all final circuits in a RCBO with a board charge?

 

[The Ghost]

max zs for this circuit will be exceeded.

That alone would be more than adequate reason for intervention of the DNO, and not being able to work on the installation until they do. HSE for me!

 

[sparksburnout]

The earth is in place and functioning.

This Lazy, Cavalier attitude worries me, frankly.

 

[7029 dave]

IMO, this is the DNO's baby, and if they are trying to bum the problem off to the Electrician doing works,I would wack a rod in, up front 100ma rcd .

 

[sparksburnout]

IMO, this is the DNO's baby, and if they are trying to bum the problem off to the Electrician doing works,I would wack a rod in, up front 100ma rcd .

Why? Would you be paying for it? The DNO should sort it, the end-user should not have to fork out for it.

 

[7029 dave]

Why? Would you be paying for it? The DNO should sort it, the end-user should not have to fork out for it.

I agree with you, but if the DNO refuse, what do you do?, as Essex says it's existing leave it, you did not like that.

 

[The Ghost]

Oh yeh Its reg 24 ESQCR by the way...
24.—(1) A distributor or meter operator shall ensure that each item of his equipment which is on a consumer’s premises but which is not under the control of the consumer (whether forming part of the consumer’s installation or not) is—

(a)suitable for its purpose;

(b)installed and, so far as is reasonably practicable, maintained so as to prevent danger; and

 

[sparksburnout]

I agree with you, but if the DNO refuse, what do you do?, as Essex says it's existing leave it, you did not like that.

It's out of spec. Make them sort it out, why shouldn't they? Why should we accept that they should not?

 

[Ian1981]

So what is your Zs on say the RFC?
With the 80% rule of thumb? 1.06 so would not meet required disconnection time as already alluded to.

1.10 ohms with the temperature correction however at maximum cable operating temperature of 70 degrees the maximum zs will be 1.37 ohms.
If it is too high at 1.10 ohms then it’s obviously too high at maximum operation temperature.
Will it ever reach 70 degrees?
In most cases probably not.

 

[Nigel]

So what is your Zs on say the RFC?
With the 80% rule of thumb? 1.06 so would not meet required disconnection time as already alluded to.

Assuming a RFC would have RCD protection, in this instance you would record the max Zs of the RCD, which would be 1667 ohms.

 

[Nigel]

That alone would be more than adequate reason for intervention of the DNO, and not being able to work on the installation until they do. HSE for me!

Well not really no. What if you had a Ze of 0.85 and wanted to install a 300A distribution corcuit in.

How would you comply with max permitted Zs?

 

[Nigel]

This Lazy, Cavalier attitude worries me, frankly.

Listen just because you do not know how to deal with this situation does not give you the right to throw about comments like that.

The op has reportes to he DNO and they have given an answer.

So you, as a contractor can either play phone call tennis for weeks on end or put the installation right with what you have.

I would choose to put it right, get paid and move on.

 

[Ian1981]

Well not really no. What if you had a Ze of 0.85 and wanted to install a 300A distribution corcuit in.

How would you comply with max permitted Zs?

Use a mccb micrologic 2.3 OCPD and set your magnetic tripping time.

 

[Nigel]

Use a mccb micrologic 2.3 OCPD and set your magnetic tripping time.

And what would the max permitted Zs be?

 

[Ian1981]

And what would the max permitted Zs be?

Lower than your imaginary 0.85 ze scenario you have just given
If you’ve got a ze that high then you’ve got bigger things to worry about than installing a 300 amp circuit as no doubt this is a commercial/industrial environment

 

[sparksburnout]

Assuming a RFC would have RCD protection, in this instance you would record the max Zs of the RCD, which would be 1667 ohms.

Oh, resorting to RCD protection now are we? Sloppy, very sloppy.