80% values when and why?

[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.

 

[Leesparkykent]

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.

 

[peterdaniels]

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.

 

[Leesparkykent]

NICEIC forms have lots of stuff that's not on the IET's model forms that you have to fill in that's not really relevant. They say max Zs permitted by BS7671 the values in BS7671 are 100% values so that's what you put on them. Even though you should still verify that 80% Zs values will still be achieved.

 

[peterdaniels]

Yeah that's what I was thinking. Record max bs7671 values on paperwork but apply 80% values to results achieved.

Cheers

 

[Leesparkykent]

Yeah that's what I was thinking. Record max bs7671 values on paperwork but apply 80% values to results achieved.

Cheers

Yeah bang on

 

[The Ghost]

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.

 

[Nightmare1308]

Yeah bang on

I know this is old post but was looking for the reason . I was working to the (Ambeinet temp+230v)÷(20degrees +230) = factor.

That aside talking about Max permitted
If you have an RCD and its 300mA and it's protecting the board as an Incommer won't this then mean they will all have 166ohms?

 

[Electro-tech]

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.beama.org.uk/static/uploaded/8bb768e3-03b7-49c6-bee56019c10e1c7a.pdf

 

[Nightmare1308]

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.

 

[Electro-tech]

Table 41.5 of BS 7671 page 70 only.

 

[Nightmare1308]

Table 41.5 of BS 7671 page 70 only.

Based on that you put the RCD as max permitted.
Was the Beama document not above the BS7671 as I didn't see a response?

 

[Electro-tech]

Based on that you put the RCD as max permitted.
Was the Beama document not above the BS7671 as I didn't see a response?

The BEAMA document was for a reference to your question not an extract from BS 7671 .

 

[newfutile]

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?

 

[Nightmare1308]

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.