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Hi Everyone,

I would like to confirm a question I saw the other day regarding working out the residual current on an RCD

The question was similar to the following
An earth electrode has been measured and the following readings were taken
1) 21 Ohms
2) 21.02 Ohms
3) 20.8 Ohms

What is the maximum residual current RCD that can be used on the circuit above.

I assume the test was obtained by test method E1 using a dedicated earth electrode.

reference to 411.5.3 (ii) Ra x I<50 V (apologies not sure how to get the correct symbols. lol would have thought it would be simple like inserting a smiley face ;))

I worked it out to 2000mA but not sure if it was the right choice, as it's not widely used especially in domestic installations.

other choices were from Table 41.5 in the regs.

what are your thoughts?
 
Firstly,where and what,is the installation application?
A heavy-water workshop in China,may have differing requirements...
 
Solely considering using an RCD for fault protection the calculation you have made seems accurate. If there are any other considerations of the installation that may limit the value of the RCD protection required then these should also apply, but from a purely fault protection approach the disconnection times will be met if a 2A RCD is functioning correctly.
From a practical perspective 100mA would be the most cost effective selection and 500mA the highest non adjustable RCD that would be readily available should it require a non adjustable RCD.
 
Solely considering using an RCD for fault protection the calculation you have made seems accurate. If there are any other considerations of the installation that may limit the value of the RCD protection required then these should also apply, but from a purely fault protection approach the disconnection times will be met if a 2A RCD is functioning correctly.
From a practical perspective 100mA would be the most cost effective selection and 500mA the highest non adjustable RCD that would be readily available should it require a non adjustable RCD.

Is the max RCD rating 100mA when an earth electrode is used?
 
Firstly,where and what,is the installation application?
A heavy-water workshop in China,may have differing requirements...
Agree with you, that information wasn't in the question. All the question asked for was the maximum residual current for resistance given taken from earth electrode
 
Is the max RCD rating 100mA when an earth electrode is used?
The question was very vague, it didn't mention where or how the installation was used. Looking through the regs I see that 300mA is the Maximum for Agricultural installations. Which makes me think that although 2000mA is possible through calculation it might have been incorrect
 
Is the max RCD rating 100mA when an earth electrode is used?

The question was very vague, it didn't mention where or how the installation was used. Looking through the regs I see that 300mA is the Maximum for Agricultural installations. Which makes me think that although 2000mA is possible through calculation it might have been incorrect
I am not aware of any limitation on the rating of the RCD where it is used solely for fault protection on a TT system, but I would await further enlightenment.
I might answer the question with "An RCD with a rated tripping current of 2000 mA would be technically permissible to comply with the requirements for fault protection by Automatic disconnection of supply on a TT installation. A significantly lower rating may well be required for other purposes within the installation."
 
This type of question recently came up in my 2391-52, and I can confirm 2000mA was the answer they were looking for. If the question doesn't mention agricultural (or any other details) it's safe to assume that they are looking for you to be able to find and use the formula correctly more than looking for particular instances where it won't be possible/applicable etc.
 
In which case,the OP should have stated the fact,the question does not list any further details,which could or would,affect the choice of device.
We do not even know the examination basis,of the question...or the country,supply characteristics,etc

I will give you an example...What is the longest one you have ever seen?

...I will await the correct answer...;)
 
The OP did inherently say that Peg, else he would have posted more information. No more information posted probably means that's all he had to go on
 
I'd like to take the opportunity to thank you all for the feedback, and appreciate you guys for taking the time to read my post.

...You haven't told us how long it is...;)
 

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