Max Zs on certification

[Dan007]

Please can people clarify what value they insert in the Max Zs field on certification.

I've always input 100%. Currently in disagreement with another person as they are specifying 80%.

Done my 2391 in 2008 and I was taught and told it's always 100% and it's down to the QS to interpret the results and ensure they fall with 80% allowing for correction factors.

Third argument is that in GN3 and OSG they specifically state 80% - however I always understood these to be quick reference pocket guides while on site.

In the mentioned publications and BBB the model forms show 100% in the compelte fields for examples. Secondly the small print at the bottom makes reference to the Publication and values in Chapter 41 aka BBB and 100% values.

Your thoughts please people

 

[Nigel]

Anyways, just as an aside, been called across to my vacant house up for sale. Neighbour has just had a small fire, FB in attendance. Water everywhere. Laptop in upstairs bedroom appears to be the culprit.

Too high Zs values, or dodgy lithium batteries?

Dildo left on charge.

 

[SparkyChick]

Anyways, just as an aside, been called across to my vacant house up for sale. Neighbour has just had a small fire, FB in attendance. Water everywhere. Laptop in upstairs bedroom appears to be the culprit.

Too high Zs values, or dodgy lithium batteries?

I hope it's not done any damage to your place Middy.

 

[Ian1981]

Midwest installed the socket it was plugged into
Run Midwest run!

 

[Midwest]

Midwest installed the socket it was plugged into
Run Midwest run!

Fortunately, I never did any work in the place

However, once the jungle drums started, it's amazing the amount of local trades persons in their vans were driving by.

 

[Midwest]

I hope it's not done any damage to your place Middy.

Nope, across the road. Their Xmas ball'sd up though.

 

[SparkyChick]

Nope, across the road. Their Xmas ball'sd up though.

That's good for you, was everyone in the house OK?

 

[Midwest]

That's good for you, was everyone in the house OK?

Yep. Luckily, family downstairs having tea, when smoke alarms went off.

 

[SparkyChick]

Yep. Luckily, family downstairs having tea, when smoke alarms went off.

That's good then... much better to have a disaster for Christmas than a tragedy.

 

[Marcus Vaughan]

But it is operating in time.....the measured Zs of my example of 1.2 on a 32A ring circuit....is below 1.37 Max Zs required to make it operate on time.

As others have said - it may not disconnect in time.
The reg group you want is 411.3.2 - automatic disconnection in case of a fault.
If you need more references from the BBB then under each of the 'Max Zs tables' there is "if the conductors are at different temperature when tested, the reading should be adjusted accordingly. See appendix 3."

If you read appendix 3 - it's all there - including the 'rule of thumb' for temperature correction.

 

[Generator]

The column for 'max Zs' should surely contain the limiting value being worked to, as a reference for the column containing the actual reading obtained. If the conductors are at full load working temperature, the table values would apply. If at ambient, then you are referred to appendix 14, for methods of correction of the maz Zs value.
The 80% factor is applied NOT to the table value, but to the calculated value of V0 / Ia which is the fault current which causes a trip within the requisite time. Funnily enough, for a 32A Type B breaker, the above calc at 230V, 160A is 1.15 ohm, and 0.8 x the table value of 1.44 is also 1.15 ohm ( 1.152).
Does that work for all of the tables and values?
Anyway, I still think the 'max' column should contain the Actual limiting value being worked to.

 

[Deleted account]

You missed out Cmin factor.

 

[Generator]

Quite right Pete - forgetting about Am't 3! - U0 x 0.95. So the rule of thumb ought to be 76% then? I'm on 240, nearly 250 - so that drops my dis time from 0.4 to 0.2 secs ( no difference for a Type B tho ).
They probably class my Uo as 230! It messed my PV inverter about tho, till I upped a few settings.

 

[PatH]

Please can people clarify what value they insert in the Max Zs field on certification.

I've always input 100%. Currently in disagreement with another person as they are specifying 80%.

Done my 2391 in 2008 and I was taught and told it's always 100% and it's down to the QS to interpret the results and ensure they fall with 80% allowing for correction factors.

Third argument is that in GN3 and OSG they specifically state 80% - however I always understood these to be quick reference pocket guides while on site.

In the mentioned publications and BBB the model forms show 100% in the compelte fields for examples. Secondly the small print at the bottom makes reference to the Publication and values in Chapter 41 aka BBB and 100% values.

Your thoughts please people

I have found so many inconsistencies and mistakes in the on site guide that it can't be relied on. Take a look at the test and inspection sheets in the appendix section. Some of the values inserted are incorrect and the advice notes don't match the forms. I would always suggest that you use your experience and knowledge in any conundrum. In this case I would enter the 80% value as this is the worst case scenario in any instance