Discuss 80% values when and why? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

Welcome to ElectriciansForums.net - The American Electrical Advice Forum
Head straight to the main forums to chat by click here:   American Electrical Advice Forum

P

peterdaniels

I know that the 80% values are applied in regards to operating temperature etc but NIC paperwork asks for the max permissible values of bs7671.

So when do you guys apply the tabulated values?

I've done commercial condition reporting in the past with a company that used their own test sheets and they always applied 80% values.

Anyone shed their opinion on this? I tend to prefer the 80% values as a precautionary tactic and to cover myself.
 
To account for conductor operating temperature at 70 degree C. When testing the conductors will be near room temp. The resistance increases with temp by a factor of 0.004 per degree. If the room temp is 20 degree C there is a 50 degree C difference between room temp and conductor operating temp when fully loaded. 50x0.004=0.2 which equates to 20 percent. Reducing the 100 percent values by 20 percent is the same as multiplying by 80 percent. You should be verifying that the Zs meets the 80% values other wise you may not be achieving the required disconnection times due to conductor operating temperature and consequently ADS.
 
Cheers, yes I'm aware of the difference in temperature effect on Zs etc. Not that I would ever expect to see a conductor operating at 70c! I was more interested that the nic paperwork asks for max permissible zs of bs7671 and not Gn3 and because of this I was wondering which values you enter.
 
I must say I have often seen 80% values entered in the Max Zs column on schedule of circuits. I think the above fudges slightly the temperature (Ct) factor applied in design of circuit and max ccc. The 80% "rule of thumb" applied to the Zs value is a safety factor quite independent from C factor calculations applied to cables as I understand it. Although I can see the reasoning in @Leesparkykents' statement having said the above.
 
The maximum earth fault loop impedance values for RCDs are given in Table 41.5 of BS 7671 page 70.. For a 30mA RCD the tabulated value is 1667Ω. The need for accurate test results of less than 1.0Ω is therefore irrelevant in these situations.
https://www.bea

You would still need to enter a value and the max.
If the RCD is there for fault or fire it would be based on the 50v÷I Delta N. (if we are working on 50v touch)
Is that document shared above the BS7671?
Therefore max permitted would be 167 for all the circuits protected by it.

I'm aware the GN3 mentioned 200 ohms. That's why I mentioned a 300mA for ease of the question.
 
I happen to disagree on this and i use 80% values on my certificates and reports. I expect many to consider this wrong .my reasoning is the maximum allowed at the time (and temperature ) of testing is 80 %.

Also i would not use (RCD) values unless a rcd is for fault protection (ie:TT). Otherwise how can you be certain a line-neutral fault will disconnect in 0.4 or 0.2 seconds?
 
I happen to disagree on this and i use 80% values on my certificates and reports. I expect many to consider this wrong .my reasoning is the maximum allowed at the time (and temperature ) of testing is 80 %.

Also i would not use (RCD) values unless a rcd is for fault protection (ie:TT). Otherwise how can you be certain a line-neutral fault will disconnect in 0.4 or 0.2 seconds?
As much as I tend to want to agree the form asks for the "max".
This is a designers responsibility to provide and design for.
If you are completing all parts its down to you to design the system before construction which in that case that detail would already be calculated and provided for the system then constructed to and inspection and testing completed comparing to the design to verify the results.
The max Zs etc are within the skill set of the person testing to identify/compare the results.
 
As much as I tend to want to agree the form asks for the "max".
This is a designers responsibility to provide and design for.
If you are completing all parts its down to you to design the system before construction which in that case that detail would already be calculated and provided for the system then constructed to and inspection and testing completed comparing to the design to verify the results.
The max Zs etc are within the skill set of the person testing to identify/compare the results.
The form asks for the maximum allowed Zs so do you put the absolute limit down or the 80% value
It's a like the minimum IR value at 1MΩ how close to the limit is acceptable
 
The footnote on the model schedule of test results allows us to use values other than those stated in chapter 41, as long as we mention the source, eg the on-site guide.
So we can use the 80% values for maximum permissible Zs as long as this is made clear on the schedule.
 
The footnote on the model schedule of test results allows us to use values other than those stated in chapter 41, as long as we mention the source, eg the on-site guide.
So we can use the 80% values for maximum permissible Zs as long as this is made clear on the schedule
It does which is added to remarks (based on model forms).
Few regulations to take into consideration,
411.4.202
411.4.204.
Manufacture data
or by calculation
many ways but Importance its no less than requirements set out by BS7671.

Personally not every system is the same so I review it each time.

Without alot of research I tend to keep inside the regs due to the issue of the following.
Where people think SRCD sockets were complaint for additional protection when they are not unless protected upstream by 30mA and its still under review, but yet a governing body releases a tech magazine and people thought they were without researching it.
 

Reply to 80% values when and why? in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

OFFICIAL SPONSORS

Electrical Goods - Electrical Tools - Brand Names Electrician Courses Green Electrical Goods PCB Way Electrical Goods - Electrical Tools - Brand Names Pushfit Wire Connectors Electric Underfloor Heating Electrician Courses
These Official Forum Sponsors May Provide Discounts to Regular Forum Members - If you would like to sponsor us then CLICK HERE and post a thread with who you are, and we'll send you some stats etc
This website was designed, optimised and is hosted by Untold Media. Operating under the name Untold Media since 2001.
Back
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website. For the best site experience please disable your AdBlocker.

I've Disabled AdBlock