Max Zs and (0.8) rule of thumb

[haptism]

Hi all, could anyone please explain the necessity for multiplying cpd max Zs values (tables 41.2 ect of the byb) by the rule of thumb value of 0.8 ?

Is it to allow for discrepancies in tripping times (for example a C type breaker trips between 5 - 10 times its rated current) ?

Im assuming that adhering to the rule of thumb value (which evidently lowers the Zs), is a good practice benchmark to aim for, rather than a requirement (or the values in the tables would be different) ?

Thanks, I bought a megger mft last week and its great !

 

[Leesparkykent]

To account for conductor operating temperature at 70 degree C. When testing the conductors will be near room temp. The resistance increases with temp by a factor of 0.004 per degree. If the room temp is 20 degree C there is a 50 degree C difference between room temp and conductor operating temp when fully loaded. 50x0.004=0.2 which equates to 20 percent. Reducing the 100 percent values by 20 percent is the same as multiplying by 80 percent.

 

[Darkwood]

The stipulated values for Zs in the BS7671 have not been factored for room temp' so the 0.8 was a ball park figure to achieve this, the on-site guide however did factor the 0.8 value for you hence the different values, so it depends where your getting your Zs values from.

Your BS7671 tables are values set for 70c but when you test your circuit usually at room temp' the resistance of the cable would be lower than loaded conditions to the max of the cables operational value, the recent amendment 3 has added confusion further by allowing for variable voltage that can exist while still in legal limits.

BUGGER! ... lee got there first, don't you have a bed to get to

 

[MisterE]

Hi, it's to compensate for the temperature of the cable when it's being measured.
The BYB tables are max Zs of the cable when under load.
I'm sure someone else will be able to provide the "full" explanation.

It must be a typing race at this time of night - beaten to it..........

 

[haptism]

To account for conductor operating temperature at 70 degree C. When testing the conductors will be near room temp. The resistance increases with temp by a factor of 0.004 per degree. If the room temp is 20 degree C there is a 50 degree C difference between room temp and conductor operating temp when fully loaded. 50x0.004=0.2 which equates to 20 percent. Reducing the 100 percent values by 20 percent is the same as multiplying by 80 percent.

Okay thanks LeesparkyKent;

 

[haptism]

Thanks all

 

[haptism]

The stipulated values for Zs in the BS7671 have not been factored for room temp' so the 0.8 was a ball park figure to achieve this, the on-site guide however did factor the 0.8 value for you hence the different values, so it depends where your getting your Zs values from.

Your BS7671 tables are values set for 70c but when you test your circuit usually at room temp' the resistance of the cable would be lower than loaded conditions to the max of the cables operational value, the recent amendment 3 has added confusion further by allowing for variable voltage that can exist while still in legal limits.

BUGGER! ... lee got there first, don't you have a bed to get to

Why they have different values in different books is added confusion;

I take it you mean variable supply voltage (220 - 240)

So..... if I take Zs values from BS7671 (3rd Am), what factor would I need to use ?

I hope any of my future Zs's dont come to close to the max or im going tobe well confused (depending on what book im looking for)

 

[Risteard]

So..... if I take Zs values from BS7671 (3rd Am), what factor would I need to use ?

These values are for the Designer of the installation. So when designing he may compensate for temperature by using a mutiplier (e.g. * 1.2) to the conductors to allow for this.

You are looking for verification figures to verify that the installation has been constructed in compliance with the design (and therefore the design data should be available).

 

[Darkwood]

Why they have different values in different books is added confusion;

I take it you mean variable supply voltage (220 - 240)

So..... if I take Zs values from BS7671 (3rd Am), what factor would I need to use ?

I hope any of my future Zs's dont come to close to the max or im going tobe well confused (depending on what book im looking for)

Supply voltage can not exceed 6% below or 10% above which equates to if I remember 216v - 253v, the amendment 3 now takes this into consideration, if you are testing at room temp' and taking your Zs values direct from the BYB then you apply the 0.8 factor, if the temp' during testing is different, say in winter in an unheated property then you need to apply the applicable factor for that temp' you test at.

If you are taking your values from the on-site guide then they don't need adjusting if your testing at room temp, if however your are testing at other temp' then you need to adjust these values from room temp' to your test temp, IE the difference in degree's from room to said temp'.

 

[haptism]

Great Darkwood, thanks. I tested R1 + R2 for my house lighting circuit earlier; while all the results fell under Max Zs for breaker values (even thou the cpd be an rcbo) they were not similar; highest i found at a pendant was 0.96 ohm (upstairs) and lowest 0.15 ohm (downstairs).

I found that using the prongs onto terminals the readout fluctuated alot and had to prod a bit to get the lowest reading. I found lower results using the croc clips direct on the core in comparison to the prongs, which could account for some of the variation in results.

When testing existing installs, specifically at ceiling roses, do you use the prong on the terminal bar throughout (quicker), or go the long way round at the core(s).

Thanks

 

[LankyWill]

Lee's a clever lad

 

[haptism]

Lee's a clever lad

So are you by the sounds of it