Zs adjustment

[Chr!s]

If the circuit has already been energised, and has been at full operating temp, then you can use the max values in BS7671.
for an initial verification you should apply the rule of thumb (x 0.8) this is to allow for the increase of resistance as the circuit warms up.

BS7671 assumes cable is at 70 C, when you test the cable will be at ambient temp, adjust the test figure by the factor.

It makes no odds energized or not you will need to make a correction to your test figures.

 

[Engineer54]

When have you ever seen a cable at 70c?? The only member here that i can think of that may have, is Tony when working in the foundry's... Even then, would the insulation be standard PVC or more likely XLPE in such conditions!!

This 0.8 factor is basically a relatively new requirement, can't remember seeing this factor being applied to max Zs values in the past!! ...Having said that, with a correctly designed and installed circuit, your Zs values should be well below the adjusted figure, let alone the max figure stated in BS7671 anyway!!

 

[telectrix]

When have you ever seen a cable at 70c?? The only member here that i can think of that may have, is Tony when working in the foundry's... Even then, would the insulation be standard PVC or more likely XLPE in such conditions!!

This 0.8 factor is basically a relatively new requirement, can't remember seeing this factor being applied to max Zs values in the past!! ...Having said that, with a correctly designed and installed circuit, your Zs values should be well below the adjusted figure, let alone the max figure stated in BS7671 anyway!!

maybe with your rods, E54, but not with the usual 3ft. twigs we generally see installed. LOL.

 

[Engineer54]

I'd certainly have to agree with you on that one!!

But then i wonder how many even consider circuit Zs values on a TT system, when they are installing their magic cover-all RCD's!! lol!!

 

[johnboy6083]

I've seen a few cables running at full temp Eng 54, usually feeding pumps. One set were so hot, me and my mate were sweating just testing the building services we had installed, even though it was freezing outside, and there was no other source of heat!
admittedly though, in unsupervised installations I would be wary of designing to limits.

 

[johnboy6083]

BS7671 assumes cable is at 70 C, when you test the cable will be at ambient temp, adjust the test figure by the factor.

It makes no odds energized or not you will need to make a correction to your test figures.

I agree, hence the reference to operating temp. When testing at ambient temp, you are at the best case scenario for impedance, which is worst case for fault currents.
therefore, it's reccomended to adjust the cold values of impedance to allow for the increase due to temperature.

 

[Engineer54]

I've seen a few cables running at full temp Eng 54, usually feeding pumps. One set were so hot, me and my mate were sweating just testing the building services we had installed, even though it was freezing outside, and there was no other source of heat!
admittedly though, in unsupervised installations I would be wary of designing to limits.​

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Well all i can say to that statement, is someone made some very bad design decisions, and/or very wide of the mark cable calculations!! I can honestly say, i have never seen in the 40 odd years in the industry cables running at 70c under normal conditions!!!

 

[Chr!s]

Well all i can say to that statement, is someone made some very bad design decisions, and/or very wide of the mark cable calculations!! I can honestly say, i have never seen in the 40 odd years in the industry cables running at 70c under normal conditions!!!

Its just a fail safe approach, most cables will run cooler than the 70/90 C in normal use, Zs though is under fault condition, so when testing at ambient we need to make an adjustment to compensate. There wil be a rise in temp of the cpc under fault condition's so if we test and its boderline at ambient, its going to be outside under fault conditions when the cables will be hotter.

 

[Engineer54]

I understand all about fail safe conditions,...and under such conditions you design and install a circuit (as well as the motor/equipment) to cope with them. Having a cable running normally at 70c isn't what i would call a well designed system, not under any stretch of the imagination!! Same goes for Zs values!!

 

[Gnasher]

When have you ever seen a cable at 70c?? The only member here that i can think of that may have, is Tony when working in the foundry's... Even then, would the insulation be standard PVC or more likely XLPE in such conditions!!

This 0.8 factor is basically a relatively new requirement, can't remember seeing this factor being applied to max Zs values in the past!! ...Having said that, with a correctly designed and installed circuit, your Zs values should be well below the adjusted figure, let alone the max figure stated in BS7671 anyway!!

The 0.8 has replaced the old rule of thumb of 75% that has been around fof at least 20 years

 

[Gnasher]

Read what i've written! I've said its great if they're below the 80% but fine if they're below the bgb values!

Incorrect. If you wish to use the BGB figures you must carry out the calculations.

 

[Chr!s]

I understand all about fail safe conditions,...and under such conditions you design and install a circuit (as well as the motor/equipment) to cope with them. Having a cable running normally at 70c isn't what i would call a well designed system, not under any stretch of the imagination!! Same goes for Zs values!!

I think your missing the point LOL.

When designing for Zs your not taking the operational temp into account, you design your circuit to the worst case, this may be Line at 70 and CPC at 70, its the basic way ensuring you meet shock protection.

If you have a 32 amp radial testing at 1.44 at 20 C, at 70 C under fault condtion that will increase thus then not meeting the required disconection time, the designer may decide to go beyond at work with the derivatives of ther K value giving an even higer Factor.

 

[Engineer54]

I think your missing the point LOL.

When designing for Zs your not taking the operational temp into account, you design your circuit to the worst case, this may be Line at 70 and CPC at 70, its the basic way ensuring you meet shock protection.

If you have a 32 amp radial testing at 1.44 at 20 C, at 70 C under fault condtion that will increase thus then not meeting the required disconection time, the designer may decide to go beyond at work with the derivatives of ther K value giving an even higer Factor.

I'm not going to argue the point on this, as it may confuse newbies and trainee's!! What i will say though is don't believe, or rather take as gospel, the mechanic's of everything you read!!!

 

[monkeyelectric]

Incorrect. If you wish to use the BGB figures you must carry out the calculations.

Only if I was designing said circuit not if I'm testing already installed wiring