80% values when and why?

[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