Understanding Zs values

[Ian1981]

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?

A type b mcb will trip between 0.1-5 seconds at 5 x the rated current
So 32amp x 5 = 160 amp
So 230v x cmin 0.95= 218.5 volts
218.5/160= 1.37 ohms
1.37 at 70 degrees
1.37 x 0.8 = 1.10 ohms

 

[Gigsy]

I still do not get it, so why does the OSG say 1.1 again?

Calculation in OSG

32 x 5 = 160A
230 x Cmin 0.95 = 218.5V
218.5 x Ctemp 0.96 = 209.2
209.2 / 160 = 1.3 ohms

 

[Ian1981]

I still do not get it, so why does the OSG say 1.1 again?

Calculation in OSG

32 x 5 = 160A
230 x Cmin 0.95 = 218.5V
218.5 x Ctemp 0.96 = 209.2
209.2 / 160 = 1.3 ohms

It's 1.37 x 0.96
It's the zs your using temp correction with not the voltage

 

[Gigsy]

It's 1.37 x 0.96
It's the zs your using temp correction with not the voltage

1.37 x 0.96 = 1.31

OSG says 1.1

I guess what you said to start with is the reason, they are using different conductor tempertures for their calculations, or something.

Thank you for your time, why they do not print the same information frustrates me, it makes learning very confusing.

 

[Gigsy]

It's 1.37 x 0.96
It's the zs your using temp correction with not the voltage

So say if I measured Zs at 1.3ohms and looked at the table in the OSG and it said it should not be greater than 1.1ohms, should I be concerned? Or just ignore the OSG and use the values in BS7671

 

[Ian1981]

It's 1.37 x 0.96
It's the zs your using temp correction with not the voltage

1.37 x 0.96 = 1.31

OSG says 1.1

I guess what you said to start with is the reason, they are using different conductor tempertures for their calculations, or something.

Thank you for your time, why they do not print the same information frustrates me, it makes learning very confusing.

1.37 x 0.96 = 1.31
1.31/1.2 = 1.096 or 1.1 ohms
The 1.2 is to adjust back to 70 degrees
I've made it too complicated just get your zs from bs 7971 and x it by 0.8 to get your max zs when testing at site temperature

 

[Gigsy]

1.37 x 0.96 = 1.31
1.31/1.2 = 1.096 or 1.1 ohms
The 1.2 is to adjust back to 70 degrees
I've made it too complicated just get your zs from bs 7971 and x it by 0.8 to get your max zs when testing at site temperature

Thank you
You are good.

 

[Ian1981]

So say if I measured Zs at 1.3ohms and looked at the table in the OSG and it said it should not be greater than 1.1ohms, should I be concerned? Or just ignore the OSG and use the values in BS7671

If the zs is too high when testing at site temperature then it will be safe to say it will be too high at cable operating temperature.
We can't test when cable is running at operating temp which is why we adjust our values for testing at site temperatures

 

[telectrix]

the osg assumes a worst case of the cable operating at 70 degrees. except in some industrial situations, this is extremely rare. e.g. i once had a 16mm T/E running at 110A for 30 minutes. temp. did not exceed 40 degrees.

 

[Ian1981]

Thank you
You are good.

No the guys with the hnc and hnd in electrical engineering are good I just get by the best I can

 

[telectrix]

hnd in electrical engineering

i got that and i ain't that good. forgotten too much.

 

[Ian1981]

hnd in electrical engineering

i got that and i ain't that good. forgotten too much.

I bet you knew it back in the day when you needed too

 

[Ian1981]

the osg assumes a worst case of the cable operating at 70 degrees. except in some industrial situations, this is extremely rare. e.g. i once had a 16mm T/E running at 110A for 30 minutes. temp. did not exceed 40 degrees.

Make you wonder if these cables actually can run at more amps in the different reference methods given by bs 7671 or they just play it too safe?