Zs question

[bs7671]

I have been asked to work out Zs,I have the Ze value and the r1 and r2,however I cant remember if there is a temp correction factor once I have found the right circuit breakers on table 41.3.I thought it was (r1+r2)+Ze+1.2? Im not entirely sure can anyone help?

 

[Sparky Ninja]

I thought it would be...

(r1&r2)/4 = (R1&R2)

Zs = Ze + (R1&R2)

???

 

[SimpleSimon]

Sort of right, 1.2 is the correction factor used when circuit designing. This is to give worst case scenario when conductor temp rises to 70c.

 

[Graeme Harrold]

It would also be X (multiplyed by) rather than + (addition)

 

[bs7671]

Yeah but the regs have a temperature correction thing dont they,isnt it called rule of thumb?

 

[Sparky Ninja]

have you been given a temperature?

 

[bs7671]

I think its supposed to be worst case so 70 degrees.To be honest I dont really understand why its not already corrected in the regs,instead of being abient temp

 

[bs7671]

So what woud you type into your calculator If r1+r2 = 3.02ohms Ze @ origin is 0.51 ohms its a t-n-s by the way

 

[Sparky Ninja]

If you have the question there, it might be worth sharing it.. this'll get resolved sooner then.

 

[bs7671]

Its on a college madeup installation and we have been asked to calculate Zs but I think they want to catch us out thats why I am unsure of these correction factors.I know Zs=(r1+r2)+Ze but how and when does the temp correction thing come in to play???

 

[Ponty Massive]

Zs from the BRB would be the max reading for that circuit breaker in order for it to operating in time (ie 0.4 sec), however when taking these readings into account you should also take into consideration allowance for temp factor or use the rule of thumb of 80%.

Take your maximum allowed Zs and multiply by 0.8.

Is this what you mean?

 

[WayneL]

Its on a college madeup installation and we have been asked to calculate Zs but I think they want to catch us out thats why I am unsure of these correction factors.I know Zs=(r1+r2)+Ze but how and when does the temp correction thing come in to play???

You don't correct your readings - your Zs is your Zs.....that's it.

The maximum figures from The Regs need to be corrected to allow for conductor operating temperature - so you multiply these figures by 0.8 and compare to your Zs.

The OSG figures are already corrected.

 

[Jurasic Spark]

If you are designing a circuit and you want a design figure of Zs you use the mill ohms per meter values from OSG or GN3.
These are based on a temp of 20 degrees so you x them by 1.2 which converts to an equivalent reading at 70 degrees.
Now add this to your Ze and you have a design value of Zs which can be compared to max values in BS7671 which are at 70 degrees.

 

[pushrod]

Is this on a ring or a radial?

Have you been asked to check the suitability of the calculated value? - If not i would think you are looking too deeply with regard to temperature correction. If yes then temp correction for the values in the brb are needed.

 

[GrannySmiff]

You don't do any correction to the Zs reading its self ever. THe corection is the regs book Zs figures need to be multiplied by 0.8 as they are cold values and once in service temp shopuld rise which will increase resistance and make a borderline acceptable instalation over the limit and possibly not disconect in time.

You can use your onsite guide for the corrected values to save you working it out as its already been done.