Any quick calcs to get a rough min Zs allowable

[rich3009]

Any quick calcs to get a rough estimate min Zs allowable for protective devices MCB, BS-88 etc, Im thinking in an exam saves time even looking at the tables if there's a fast ball park check rule of thumb.

 

[Leesparkykent]

Do you mean max Zs?

 

[rich3009]

yes sorry of course

 

[westward10]

Takes seconds to look at the Tables.

 

[Leesparkykent]

(UoxCmin)/Ia

 

[rich3009]

Takes seconds to look at the Tables.

It's a good check though

 

[telectrix]

Zs < V/ 5In for B type. e.g.for a 32A . Zs < 230/(5 x32) i.e. 230/160 = 1.4375. ( then apply correction factors).
Zs < V/10In for C type.
Zs < V/20In for D type.

 

[rich3009]

I knew I'd seen something like this and I remember rule x10 for C and D types, anything for BS88

 

[westward10]

Not sure how these quick calculations will save you time if you have the Tables to hand, it is just something else to remember.

 

[Zdb]

Quicker just to turn to page 58 in the big yellow book.

 

[Ian1981]

Zs < V/ 5In for B type. e.g.for a 32A . Zs < 230/(5 x32) i.e. 230/160 = 1.4375. ( then apply correction factors).
Zs < V/10In for C type.
Zs < V/20In for D type.

You’ve forgotten cmin so it’s 1.37ohms for 32 amp type B and not 1.44

 

[telectrix]

not forgot. stated in post to apply correction factors.

anyway, cmin is to allow for voltages below 230V. what about when the voltage is > 230V. ????

 

[Ian1981]

not forgot. stated in post to apply correction factors.

Thought you meant the 0.8
I’ll apologise
Type D tho is 230x cmin /10In for 5 second disconnection time circuits and 20In for 0.4 second circuits

 

[telectrix]

agreed. was just considering the 0.4 secs, as most circuits (apart from distribution ccts) are 0.4,

 

[Ian1981]

We have to use the worst case scenario as given in the regulations or use manufacturers data.
As we know the voltage measured could be anywhere between 230-250 v or whatever really.
You know what they are like regarding changing regulations and moving the goalposts