Help with the 2391

[babyshanks]

So I have the 2391 exam next thursday. I got some old 2391 papers from eBay and one of the questions has no answer to it, and I have not been taught this in any of the evening lessons, so I was wondering if someone could help out.

The question is:
Determine the maximum operating current, in mA, for an RCD providing fault protection for a domestic TT installation, if the earth electrode resistance is 166Q. Show all calculations.


​Thanks.

 

[Plonker 3]

...............................

 

[tony mc]

I take it that 1660 ohms

Ra x I DELTA N < 50v (Equal or less than 50 v) could not get the symbol to work.

50/Ra = Idn

50/1660 = 0.0301

So a 30 mA RCD will be good.

 

[spark 68]

I agree with your method Tony mc, but the answer may be a 300mA RCD.

To the OP, have a look at section 411.5 in the BGB, this explains all, particularly the table 41.5, as you are almost guaranteed a question like this in the 2391 exam.

 

[JUD]

I read it as 166Ω. Worked it out same way though..

RA x IΔn ≤ 50V

50 ÷ RA = IΔn

50 ÷ 166 = 0.3A = 300mA

 

[tony mc]

Also remember show all of your calculations as you will get a mark for each stage!

Even if you final answer is wrong you can still pick up marks !!!

 

[babyshanks]

Nice one guys thanks. So RA x IΔn ≤ 50V is the equation I need for this? Basically ohms law. Is it alway going to be 50V?

 

[JUD]

Is it alway going to be 50V?

Yes it is

 

[telectrix]

generally. 50v is the max. touch voltage. in certain sitruations this may be 25v.

 

[spark 68]

Nice one guys thanks. So RA x IΔn ≤ 50V is the equation I need for this? Basically ohms law. Is it alway going to be 50V?

Yes, as this is the touch voltage on the earth, and it is considered dangerous to allow this to rise above 50V.

In certain highly specialised locations (operating theaters for one), this is reduced to 25V.