Max Zs Permitted by BS7671 value

[Loges1504]

Hi All,
Thank you for welcoming to this forums

I just want to clarify that the Value I need to enter 100% or 80% in the last column "Maximum Zs Permitted by BS7671"
is it 80% from the table 41.3 from 18th Edition Regs - Page 62 or the value from Pocket Guide 18

Thank you
Loges

 

[Toneyz]

You put the 100% value down in the column for max permitted Zs by BS7671.

 

[Loges1504]

Great, Thank you..

 

[telectrix]

You put the 100% value down in the column for max permitted Zs by BS7671.

agree.

 

[Loges1504]

Thank you all.. it is a great help.

 

[telectrix]

Thank you all.. it is a great help.

if you were from essex, i' would not have bothered.

 

[Loges1504]

Thank you.

 

[Deleted account]

I was taught to put the 80% down. And that’s what I’ve always done.

This was discussed in a previous thread and some said 100% others 80%.

I don’t think it really matters as long as you know which your using on the comparison to the actual Zs read and you know what your doing.

 

[Deleted member 9648]

If a 30ma RCD is incorporated along with the overcurrent device then technically the maximum permitted Zs is 1667 ohms, but you almost never see this applied to TN systems, most incorrectly use the max Zs for the OCPD.
411.4.5.......411.4.204 and table 41.5 apply to TN systems.

 

[Strima]

If the certificate asks for "Maximum Zs Permitted by BS7671" then you put down the 100% figure from the book as that is what it is asking for.

 

[Deleted member 26818]

As Strima says, you put down the value found in BS7671.
This is so the customer gets confused when the measured value is less but it still fails.

 

[telectrix]

As Strima says, you put down the value found in BS7671.
This is so the customer gets confused when the measured value is less but it still fails.

but then you confuse them even more by telling them it's oK, with RCD it's 1667.

 

[UNG]

For me you should use the 80% value

The maximum permitted Zs value is 80% of the maximum allowed value

As the certs asking for the maximum permitted value then the 80% value should be used

 

[telectrix]

BUT 80% is not max. it's corrected assuming cable will be fully loaded and at 70 deg.C. this will never happen. cables will be at least 30% overloaded and breaker will trip long before that temp. is reached (assuming circuit has been designed in compliance with 7671). tin hat on.

 

[Deleted account]

BUT 80% is not max. it's corrected assuming cable will be fully loaded and at 70 deg.C. this will never happen. cables will be at least 30% overloaded and breaker will trip long before that temp. is reached (assuming circuit has been designed in compliance with 7671). tin hat on.

Absolutely agree. I don’t understand why we even use 80%. It is overkill.
I think 90-95% would be adequate just to allow for design flaws and anomalies.

However the Cmin factor that gives 95%. So I recon that should be sufficient.

Obviously there may be very good reasons 80% is used. Maybe a figure plucked out of the air. I don’t know. Any ideas?

 

[telectrix]

80% = a 20% increase in IET pension fund.
Cmin. = 5% increase in IET pension fund.
18th Edition = 150% increase in IET pension fund

 

[Deleted account]

I wish the winner button was still there. You’d have just got one.

 

[UNG]

BUT 80% is not max. it's corrected assuming cable will be fully loaded and at 70 deg.C. this will never happen. cables will be at least 30% overloaded and breaker will trip long before that temp. is reached (assuming circuit has been designed in compliance with 7671). tin hat on.

The maximum ALLOWED value of Zs is the 100% value
The maximum PERMITTED value of Zs is the 80% value

Pretty clear cut IMO and provides a reasonable margin of error for any changes in the environmental or installation conditions

I would rather work to the 80% value than be trying to talk my way out of a stay at one of Her Majesties hotels because I used the 100% value and it all went wrong

 

[Deleted account]

I think we all know we work to the 80%. It’s just what value is put on the sheet.

As I said previously. What is on the sheets doesn’t matter as long as the comparison is with the 80%.

 

[Deleted member 9648]

I would rather work to the 80% value than be trying to talk my way out of a stay at one of Her Majesties hotels because I used the 100% value and it all went wrong

It wouldnt go wrong with the 100% value, the difference in disconnection time would be marginal or non-existant in real, not calcualted situations. Nobody will ever be prosecuted for using the maximum value of Zs clearly written in the regulations.