meaning of 'design' value for max Zs?

[gmdaly]

Hi all, I have an assessment exam question that asks for the design value for a 32A 60898 MCB max Zs. By 'design' value is this the regs tabulated value (working temp) or the lower value 80% for testing in ambient?. With no other information the word design is throwing me as it seems like it could be either.

Thanks for the help.

g

 

[Julie.]

Design, would be the value from the reg book NOT the OSG , so in the case of a 32A type B 1.37 ohm not 1.1 ohm.

(Think I got the correct numbers - but you need to know what type, just 32A is insufficient information)

32A could be:
1.37 ohm / 1.1 ohm type B
0.69 ohm / 0.55 ohm type C
0.34 ohm / 0.27 ohm type D

From memory


The system is designed to run at the running temperature, so the Design Zs must be the value at running temperature.

If you measured 1.1 ohm when cold, it could end up at 1.37 ohm when at running temperature.

 

[gmdaly]

Design, would be the value from the reg book NOT the OSG , so in the case of a 32A type B 1.37 ohm not 1.1 ohm.

(Think I got the correct numbers)

The system is designed to run at the running temperature, so the Design Zs must be the value at running temperature.

If you measured 1.1 ohm when cold, it could end up at 1.37 ohm when at running temperature.

thanks so much. I thought so but then started second guessing myself.

 

[Julie.]

thanks so much. I thought so but then started second guessing myself.

If you do have the regs to hand, cross check my figures as I am not near a book, and it's just memory - I edited it to add the type C and D.

 

[pc1966]

The values at working temperature ought to be based on:
Zs = 0.95 * Uo / (In * A)
Where A = 5 for B-curve, 10 for C-curve and 20 for D-curve (i.e. ratio of upper magnetic trip to nominal current) so 32A C-curve is:
Zs = 0.95 * 230 / (32 * 10) = 0.6828125
Near enough 0.69 ohms.

As @Julie. says you need to meet this at the max working temperature, and for most systems that is taken as 70C with your DC measurement or design values typically at 20C, leading to a 20% rise in resistance hot versus cold. The OSG tabulates these 0.8 factor cold reading values to make it simpler when inspecting to know if Zs is likely met when hot.

Other conditions might need different factors but for most work the 80% when cold is fine. For example, cables run in very hot environment (e.g. MICC in steel works, etc) might need to reduce the cols Zs further, or a cable that is large in CCC because you need to keep VD down might run cooler so you are fine with Zs closer to the regs table, etc.

 

[pc1966]

the Design Zs must be the value at running temperature.

This is the TL;DR answer!

 

[Markyd]

LITTLE TIP that might help you in future
For your 100% Zs and a Type B take 44 and divide it by the size of the breaker eg 44 divided by 32 =1.375 .....For a Type C as above then divide by 2...For a Type D same again then divide by 4.
For your 80% values divide by 35 for a Type B , 35 divided by 32 =1.093 round it up to 1.1 and again for a Type C divide the answer by 2 and a Type D divide the answer by 4
This works for all your 60898 and 61009 breakers... 32a, 20a,16a.10a and 6a

 

[pc1966]

LITTLE TIP that might help you in future
For your 100% Zs and a Type B take 44 and divide it by the size of the breaker eg 44 divided by 32 =1.375 .....For a

Yes, that is a short way of doing the same calculation:

Zs = 0.95 * 230 / (In * 5) = (0.95 * 230 / 5) / In = 43.7 / In

So more or less 44/In, etc/