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

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.
 
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 .
 
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
 
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
 

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

YOUR Unread Posts

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