Discuss Max Zs Permitted by BS7671 value in the Electrical Forum area at ElectriciansForums.net

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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
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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.
 
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.
 
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?
 
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
 
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.
 

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