Max Zs in OSG and the Regs

[tank]

Hello all, i'm currently doing level 3 2330 and am getting a little confused with the different max Zs values in the OSG and the Regs. I'd appreciate any advice .

Would i be right here

I use the max Zs values in the OSG for installation design as these are lower to account for the reduced resistance of cold conductors, as opposed to the operating temperature max Zs values in the regs.

ie. Zs = Ze+[(R1+R2) x multiplier x length
1000

Does this mean that when subsequently doing a live Zs test, I would check the results against those in the regs instead of the OSG. Or am i wrong to think that an energised installation means that the conductors are at operating temperature? Should i stick to the OSG when testing too?

 

[telectrix]

when you perform your Zs test, the conductors should be at ambient temperature with no loads applied. as you say, in use, the temperature will rise according to load, up to a max. of 70deg. which is why the osg uses 80% factor. seriously, though, i have never seen a cable approach anywhere near 70deg. in normal use, and i always enter the brb ( now bgb) figures on test sheets.

 

[MarkieSparkie]

The Zs values in BS7671, Tables 41.2, 41.3, 41.4 and 41.6 are maximum installation design values at the normal cable operating temperature (70° C for PVC and LSHF cables). When testing at ambient temperature these values can be de-rated by applying the 80% Rule of Thumb to compensate for the change of temperature, as detailed in BS7671, Appendix 14.

The Zs values in OSG, Appendix B and GN3, B are the maximum test values for PVC and LSHF cables at an ambient temperature of 10°C. These values are not calculated by the simple rule of thumb. They are calculated using a more complicated method (see GN3 B1 and EIDG) that compensates for conductor temperature and yields a more accurate result. These tables are useful in situations where the rule of thumb yields marginal non-compliance, often referring to the GN3, B or OSG, Appendix B tables, reveals that by using the more accurate method of calculation the device actually complies with the regulations.

 

[tank]

thanks for the replies lads, I'm being given plenty to think about and i'm now back into my books. a couple of things though,

telectrix, what do you mean by you always enter the big green book figures on the test sheet? are max permitted Zs values required on the test sheet as well as the test result?

markiesparkie, so in design, to use max Zs of BS7671, Tables 41.2, 41.3, 41.4 and 41.6, i would need to calculate temperature coefficients , alternatively just use 80% and no coefficients?

also GN3 OSG are the actual on site testing Zs maximum values, this helps a lot. We were told at college that marginal non-compliance with the OSG would lead us back to the regs for a more complicated calculation method to be carried out. but this isn't this case? i suppose my teacher knows a lot, just not everything.

once again thanks for the help

 

[MarkieSparkie]

Hi tank, I think a rogue comma in my earlier post may have misled you slightly (now edited). So I've tried to shed a little more light on the separate design and test applications of the max Zs tables in BS7671. I hope this helps.

Designing (using the simple method):
When designing an installation circuit it usual to assume the worst case, initially at least. When calculating the maximum Zs for the circuit, the worst case condition would be if the normal cable operating temperature was taken to be the maximum permitted normal operating temperature for that cable. The designer calculates Zs based on this worst case and checks that the value obtained is less than the maximum allowed Zs for the protective device in BS7671 tables 41.2, 41.3, 41.4 or 41.6 as applicable.
Note:- There is no need to apply temperature compensation in this case.

Testing:
The inspector tests a circuit and obtains a Zs value, then checks that the value measured is less than the maximum allowed Zs for the protective device. Because the tests are conducted with the cable at ambient temperature and not at it's normal operating temperature, this change in temperature must be compensated for. A simple way of compensating for the change in cable temperature is to apply the 80% Rule of Thumb to the tabulated maximum Zs values given in BS7671 tables 41.2, 41.3, 41.4, or 41.6 as appropriate.
When use of the 80% Rule of Thumb yields "marginal non-compliance" the tables in OSG or GN3 can be used as these are more accurate. These tables assume an ambient temperature of 10°C, if the tests were performed at a different ambient temperature a correction factor must be applied to these values to compensate.
If these tables still indicate "marginal non-compliance" the inspector could do a full maximum Zs calculation with even greater accuracy in order to try and achieve compliance, however, the calculation is long winded, torturous (particularly when line and CPC are not the same size) and of diminishing return. A practical solution is usually far quicker than the full calculation.

 

[ackbarthestar]

Usually, certificates specify both the maximum values for Zs and measured test values.

However, the BS7671 tables are design values whereas the OSG is used to check for measured values.

The difference being
BS7671 takes a general account of the multipliers for ambient and operating temperatures

Zs = Ze +(R1 +R2)/100 * L * M

and the measured test results, when compared against BS7671 which are then adjusted by a nominal 80% because there are no temperature factors for operation in both normal and fault conditions when the tests are recorded.

Zs(test) <= Zs(BS7671) *0.8