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.

 

[UNG]

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

 

[loz2754]

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.

 

[Nightmare1308]

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.

 

[buzzlightyear]

I have had this out with the NICEIC inspector, what should I put down on the test sheet, he says 100% not 80%, they call it from cold

 

[Nightmare1308]

I have had this out with the NICEIC inspector, what should I put down on the test sheet, he says 100% not 80%, they call it from cold

Is this from a new installation or a periodic?

 

[pc1966]

In terms of RCD Zs then your cable hot/cold change is going to be negligible to the overall acceptable value. Less than measurement accuracy for sure, unless you have a huge cable R2 and then you have something more fundamentally wrong.

The Ra value limit of 200 ohms is an attempt to make sure you have a stable earth rod behaviour. It should be lower than that under practically all cases in the UK. Some regions with very dry soil or rock substrate might have difficulties unless you drill down far, but mostly for the UK using typical incomers of 100mA (500 ohm, but 200 ohm for stability) or 300mA (166 ohm or less) you ought to meet it under all but drought conditions if you have a 2.4m (8') deep rod so it finds some moisture.

As for the Zs values to use, it should be based on the max working temperature. So:

• If you cable is CCC limited then it will run at max (say 70C) so you should be using 80% when measured cold prior to use.
• If you have a cable that is VD limited, so CCC ample, it won't run as hot so a de-rating of less might be appropriate if it is proving a close call.
• If you measured it just after normal operations (say EICR) then you could may not need derating at all as it is measured hot.

 

[newfutile]

Just to add a bit more to this: page 139 onsite guide states the tables in this appendix provide maximum permissible earth fault loops impedances (Zs) .
it goes on to state these values must not be exceeded at ambient temperature . therefore unless we preheat to 70C before we test then this is the actual real life maximum permitted Zs, further more page 145 gives values for BS 3871 , 60898 and 61009 . This say to me that for additional protection these values must not be exceeded even for (RCD) Residual Current Device .

 

[Nightmare1308]

Just to add a bit more to this: page 139 onsite guide states the tables in this appendix provide maximum permissible earth fault loops impedances (Zs) .
it goes on to state these values must not be exceeded at ambient temperature . therefore unless we preheat to 70C before we test then this is the actual real life maximum permitted Zs, further more page 145 gives values for BS 3871 , 60898 and 61009 . This say to me that for additional protection these values must not be exceeded even for (RCD) Residual Current Device .

Little bit of clarification on your comments mentioned. You mention 70 degrees which I'm guessing your basing it on a common installation being the max value and that the CCC is based on, pending the correction factors etc.
Ambient temp is based on the environment and factors again which impacts the system again as mentioned.

Max permitted is based on 80% as that's the adjust made as all conductors in the system may not be the same temperature.
Table 41.2/41.3/41.4 note 2 last comment, unless the conductors temperature is known.

120 General last paragraph based on the designer.

When the designer, designs the system he will provide all relevant data etc etc as per CDM2015. In a domestic situation this will be passed on to the contractor not the owner.
Therefore the design stage and pre construction pack is important for the process from design construction to inspection and testing.

The inspector and tester should be comparing the stages with the results he has and max permitted values set be the designer.

411.4.5 states an RCD can be used as long as it has an overcurrent protective device.
If an RCD is used the max permitted will be 41.5
This would then be the max value permitted as it will be higher than the OCPD.
It also state a vaule exceeding 200ohms may not be stable (41.5 note 2). then refers to 542.2.4 which advises of the depth of the electrode......

Appendix 3, there is a paragraph in this part which mentions if manufactures data isn't available. the data in the tables may be used. Then refers to the regulations and measured value and formular.

Technically the manufactures data should be used and this does differ from the tables on a number of brands so are people taking this into consideration or ignoring the comment in appendix 3?
Yes likely the BS 7671 is lower but the question is max permitted and down to the designer to provide.

Enjoy the communication and conversation we are having speak soon

 

[yongquan hu]

The reference test temperature is 10 census
When cable is live, worst scenario to see cable temperature increase to 70 census.

The resistance will be (1+0.004x( 70-20))/(1-0.004x(20-10))=1.2/0.96=1.25

Resistance at 10 is 1/1.25 of resistance at 70

1/1.25=0.8