Understanding Zs values

[Gigsy]

Can anyone help me understand Zs values.

In the On-Site-Guide page 60 Table 7.1 under the heading Maximum cable length, it states if

Zs < 0.35 ohms for a TN-C-S system

you can have a cable length of 107m



In BS7671 page 58 Table 41.3 it states the

Maximum Zs for a 32A CB to operate within 0.4s is 1.37 ohms.



It was my understand that Ze is 0.35 for a TN-C-S system, so why does it state Zs as being 0.35 ohms in the On-Site-Guide. Is this a mistake in the OSG, should it be Ze not Zs?



I was looking for the maximum Zs of a circuit in the ONG, but I can not find where it is, Does anyone know if the OSG has a reference to maximum Zs values, surly they have to be in there?

 

[Wilko]

Hey Gigsy - check out appendix B of OSG, Table B6 has B type MCBs (original table has errors, there's a stick on page update in mine).
I think you're right about the use of Zs on p60, should be Ze (ie Ze <= 0.35 for TNCS)

 

[Gigsy]

Hey Gigsy - check out appendix B of OSG, Table B6 has B type MCBs (original table has errors, there's a stick on page update in mine).
I think you're right about the use of Zs on p60, should be Ze (ie Ze <= 0.35 for TNCS)

Thank you, that is the table I was looking for. So that was the table that came with my OSG. I have two copies of the OSG and I have lost the correction table from both

They do not make that table easy to find do they. Thank you!

 

[Gigsy]

Hey Gigsy - check out appendix B of OSG, Table B6 has B type MCBs (original table has errors, there's a stick on page update in mine).
I think you're right about the use of Zs on p60, should be Ze (ie Ze <= 0.35 for TNCS)

In BS7671 Table 41.3, page 58 states:

The Zs for a 32A type B CB to trip in 0.4s is 1.37 ohms

The On-Site-Guide, Table B6, page 125 states:

The Zs for a 32A type B CB to trip in 0.4s is 1.1 ohms

Is this the table that is wrong?

 

[Ian1981]

Tables in on site guide for max lengths of circuits assume a maximum volt drop of either 3% for lighting or 5% for other circuits.
Example
20amp type b cb with 2.5/1.5 cpc
5% volt drop is 230 x 0.05 is 11.5 volts
11.5 x 1000= 11500
Ib x mv/a/m 20 amp x 18 mv/a/m for volt drop = 360

11500/ 360= 31.9

Table 7.1 page 64 gives max length as 31 meters
For 20amp cb type b

 

[Ian1981]

In BS7671 Table 41.3, page 58 states:

The Zs for a 32A type B CB to trip in 0.4s is 1.37 ohms

The On-Site-Guide, Table B6, page 125 states:

The Zs for a 32A type B CB to trip in 0.4s is 1.1 ohms

Is this the table that is wrong?

No its 1.37 x 0.8 for temp correction = 1.10 ohms
See appendix 14 in bs 7671
On site guide gives values with on site testing temperature adjustment

 

[Gigsy]

Tables in on site guide for max lengths of circuits assume a maximum volt drop of either 3% for lighting or 5% for other circuits.
Example
20amp type b cb with 2.5/1.5 cpc
5% volt drop is 230 x 0.05 is 11.5 volts
11.5 x 1000= 11500
Ib x mv/a/m 20 amp x 18 mv/a/m for volt drop = 360

11500/ 360= 31.9

Table 7.1 page 64 gives max length as 31 meters
For 20amp cb type b

Good maths, but why are you telling me about the maximum lengths of cables in regard to voltage drop, when the question is concerning maximum Zs values?

 

[Ian1981]

Another way is 1.37 x 0.96 to adjust to 10 degrees = 1.31/ 1.2 to adjust to 70 degree cable operating temperature is 1.10 ohms

 

[Ian1981]

Good maths, but why are you telling me about the maximum lengths of cables in regard to voltage drop, when the question is concerning maximum Zs values?

because table 7.1 which you mentioned deals with max lengths of circuits regarding volt drop

 

[Gigsy]

No its 1.37 x 0.8 for temp correction = 1.10 ohms
See appendix 14 in bs 7671
On site guide gives values with on site testing temperature adjustment

Thank you, it adds up. Wonder what temp. the values in BS7671 are based on then?

 

[Ian1981]

Thank you, it adds up. Wonder what temp. the values in BS7671 are based on then?

70 degrees max operating temperature of cables
Remember when a circuit is in use it will be operating at a certain temperature.
Bs 7671 assumes worst case scenario of max operating temperature of 70 degrees hence the maximum zs allowed

 

[Gigsy]

Another way is 1.37 x 0.96 to adjust to 10 degrees = 1.31/ 1.2 to adjust to 70 degree cable operating temperature is 1.10 ohms

It is interesting you mentioned 0.96 as a correction factor.

page 58 of BS7671 Note 1, states they used a correction factor of Cmin 0.95 to account for the potential voltage drop of the supply.

Is it a temp correction value or do you think BS7671 has accounted for Cmin and the On-Site-Guide has not?

 

[Ian1981]

It is interesting you mentioned 0.96 as a correction factor.

page 58 of BS7671 Note 1, states they used a correction factor of Cmin 0.95 to account for the potential voltage drop of the supply.

Is it a temp correction value or do you think BS7671 has accounted for Cmin and the On-Site-Guide has not?

Cmin is applied to 230 v and is 230 x 0.95 = 218.5volts
The 0.96 is to temp adjust to 10 degrees nothing to do with each other.

 

[Gigsy]

You know your stuff, your math is spot on!
You are correct in saying BS7671 gives values of max Zs permitted max operating temp is not exceeded (Note 2).
You are correct in saying that values in OSG are given based on ambient temperature.
I agree that Cmin and temperature correction values are different things.

Thank you for your help

 

[Gigsy]

So Cmin is applied to both tables then? The OSG also factors in a temperature correction factor, that is the reason the values are different?