last minute 2391 questions

[training_spark]

hi

can anyone help me with the following questions please

1) an earth electrode resistance test has been carried out using an earth electrode tester, the following results were obtained 155, 157, 161 (ohms) determine the value of Ra to be recorded on the periodic inspection report, show all calculations.

Am i right in thinking that you add all three values and divide by 3?

2)maximum test current to be applied to a 300mA rcd?

100mA?

3)forumla for calculating CSA of a conductor when they give you the mili ohms value and length of the circuit

4)One year after the completion of the office extension, one of the luminaires in the office area requires replacing.

Before undertaking the replacement of the luminarie list three items that need to be considered.

5)testing radial circuits, (cpc tests, IR, polarity, zs at furthest point, any more ?

6)Ra I*N <50V anyone have some questions using rcd formulas?

Will post some more in a bit, cheers

 

[Smugley]

Okay, I'll have a bash at a few..

1. Yes, the mean value is taken, ie the average of three. GN3 pp46-47

2. Max test current would be 1xIn, ie 300mA. (Only do a 5xIn test on 30mA RCDs)

3. Err, I'd use tables, am I being dim? Can't think of any formula other than re-arranging the equation based on the mOhm/m value.

4. Guessing as to what C&G want here. Never quite know. Like-for-like doesn't require a MEIWC so dunno if they want standard tests such as IR, Zs etc. Perhaps

- check the replacement item is suitable, ie for the environment, relevant BS number etc

- confirm the replacement item is the same rating as the old one otherwise circuit design may need to be reconsidered.

- perhaps arrange for a convenient time to isolate the circuit - might be important if the office is occupied.

5. In GN3, verification of voltage drop is listed at the end. Depends what type of testing you are doing ie: initial verification, periodic inspection..

6. ?