TT system and Zs results

[keefy]

Just finishing off cert for a TT system, got a Zs of 115 ohms throughout. will I be right in thinking the max Zs column for all circuits will be 1667 or 200?

Cheers

 

[Guest123]

Just finishing off cert for a TT system, got a Zs of 115 ohms throughout. will I be right in thinking the max Zs column for all circuits will be 1667 or 200?

Cheers

That would depend on what rating of RCD you have protecting the circuits....

The 200ohm value is the max recommended by BS 7671 for the installation Ra.

 

[keefy]

A 100ma type s at the cut out as it is a short distance from the DB and a dual 30ma RCD in the board as it is a domestic property.

Cheers

 

[MarkieSparkie]

The 30mA RCD additionally protected final circuits will have a Zs maximum of 1667 Ohms.

 

[Richard Burns]

For a non time delayed 100mA your maximum Zs would be 500 ohms, if all the circuits are protected by the 30mA RCDs then max Zs is 1667 ohms.
Ra x I delta n <= 50V
so 50V/0.03s = 1667 ohms
50V/ 0.1s = 500 ohms

The time delay causes problems for disconnection times as it will add about 0.15s to the time.

 

[Engineer54]

For a non time delayed 100mA your maximum Zs would be 500 ohms, if all the circuits are protected by the 30mA RCDs then max Zs is 1667 ohms.
Ra x I delta n <= 50V
so 50V/0.03s = 1667 ohms
50V/ 0.1s = 500 ohms

The time delay causes problems for disconnection times as it will add about 0.15s to the time.

But still well worth having as a supplementary back-up for those 30mA RCD units, on a UK installed TT system with an Ra of 115 ohms wouldn't you say?? lol!!!

 

[Richard Burns]

But still well worth having as a supplementary back-up for those 30mA RCD units, on a UK installed TT system with an Ra of 115 ohms wouldn't you say?? lol!!!

Nice low Ra like that! Well yeah I think so, you are pretty much as safe with a 100mA as a 30mA gone in 0.02 seconds, or 0.17s better than .4s whatever. (sorry, of course the regulations, forgot about them: 100mA deadly!)

 

[Rauer]

One of my mates was testing for the council and on tt systems they wanted him to link the neutral to the earth at the board ( so in theory it's a tn system) to get more "true" zs readings

 

[Engineer54]

One of my mates was testing for the council and on tt systems they wanted him to link the neutral to the earth at the board ( so in theory it's a tn system) to get more "true" zs readings

You sure they didn't mean ''False'' reading?? lol!!

 

[Rauer]

I did think to myself ......hang on a minute!!! But I suppose they would satisfy the maximum zs for a change!!!! Not quite sure how the nic etc. would like it tho, but it was the councils head electrician that told him!

 

[trev]

What would be the point of linking those conductors? Any reading given would be completely worthless

 

[Rauer]

What would be the point of linking those conductors? Any reading given would be completely worthless

Pass!!!!

 

[Engineer54]

I did think to myself ......hang on a minute!!! But I suppose they would satisfy the maximum zs for a change!!!! Not quite sure how the nic etc. would like it tho, but it was the councils head electrician that told him!

Then he needs sacking or retraining!!

Head Electrician, ....more like head Plonker!!

 

[Specialist]

After telling someone to do that I think i'd be leaning towards the Sacking option myself.

 

[i=p/u]

just doesnt make sense, dealing with what you have got is easier