max zs table in the osg

[Jonspark]

Think about it for a second, previously stated max Zs values satisfied ADS requirements, so how can you, (or should i say they) now say that they Don't??

Of all the things that needed sorting out within BS7671, max Zs values wasn't and still isn't one of them. A totally wasted Amendment opportunity all round, when you also take into account the plastic/metal CU farce!!!

your getting ur wires crossed,(sorry for the pun) all i'm saying is that exceeding BS7671 max CURRENT Zs figures is supposed to constitute a C2??? aaaahhhhhhhhhh!!!!!!!!!!!!!!!

 

[Jonspark]

I'll use the 7671 figures, you can use what ever you want.

 

[Engineer54]

the time/current graphs in BS7671 are generic. different makes of OCPDs may well give different (better) figures.

They shouldn't do Tel, if were talking about MCB's they are built/manufactured to a common standard according to the type B / C / D, also all the time/current curves shown in BS7671 are based on the slowest operating times...

 

[Engineer54]

your getting ur wires crossed,(sorry for the pun) all i'm saying is that exceeding BS7671 max CURRENT Zs figures is supposed to constitute a C2??? aaaahhhhhhhhhh!!!!!!!!!!!!!!!

I'm not getting my wires crossed at all, i know eactly what i mean!! lol!!

 

[spark 68]

They shouldn't do Tel, if were talking about MCB's they are built/manufactured to a common standard according to the type B / C / D, also all the time/current curves shown in BS7671 are based on the slowest operating times...

Well even in BS7671 there is provision to deviate as long as the same outcome or better is achieved, it tells us this in the good book itself.

Now most sparks (me included) will just follow the book because we are not qualified to know any better, this is no reflection on our ability and knowledge in any way, this is just the level we have been trained and taught to.

I have in the past worked for firms with their own design engineers, and these people do know better, and will often deviate because they have better data, or more importantly the knowledge to interpret that data to achieve compliance in ways us mere sparks cannot do, they are also signing that design off and taking responsibility for that decision, something I suspect most ordinary sparks would not be prepared to do.

 

[Darkwood]

The BYB max zs values are in assumption the cable could have a conductor temp' up to 70'c to which it is rated to be usable, when we test the conductor has vitually no load and thus will be at ambient Temp' this is why an adjustment is made for testing to allow for this difference, the OSG has already been adjusted for average ambient Temp' but the regs give values to cover many situations in industrial where conductor temps may well be higher so it give the opportunity for you to know the absolute max at 70c so you can taylor your calcs to suit.

If testing at lower or higher temp's than ambient then the OSG values need adjusting to reflect the difference and the BYB need to have the correct calc applied for the testing ambient temp.... if the cable is subject to any temp' variation say going through a warmer or cooler area during testing like say a fridge or proving room then it will need to be taken into account.

If you do not bother to adjust your values and just stick to the BYB max values then your circuit may not be meeting permitted trip times while under load especially if we have a heatwave and the ambient temps could be 15c + higher than were tested at and 35c + if you testing in a unheated building while building a snowman in your break - the cable may suffer damage in a fault which could impear its function and at worse become a fire hazard ... all because it was tested in ignorance to what you should have had drilled into you at college.

 

[Engineer54]

You could say the same for conducting IR tests at different temperatures. It basically boils down to knowing what you're doing, and knowing when to make adjustments/allowances accordingly to the environment the installation is subject to. All this arbitrary clap trap of using a 80% factor, is for the picture book readers....

EDIT... Let's be honest here, when are you realistically ever going to see an installation with cable temperatures running anywhere near 70 C in normal/seasonal conditions?? The only time i've seen it, was under fault conditions.... Bit different if say you are talking about one of Tony's steel factory furnaces etc, but elsewhere, ...Nah!! lol!!

 

[davesparks]

The column on the cirtificate where the value of max Zs is filled in is headed "max Zs permitted by BS7671"

That column doesn't exist in the model forms, it's something which is added by companies who produce their own versions of the forms.

 

[davesparks]

don't quite follow u eng, nothings changed! just saying that exceeding max zs values as given in BS7671 is a C2 is it not? or is it all FINE!!!!!!

Zs values have changed with the daft Cmin factor which has been introduced.

Exceeding the max values given in bs7671 will warrant a C2 code unless the designer of the installation has used values from another source or has other reasons to set the max Zs higher.