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# 2391-52 exam question

Discuss 2391-52 exam question in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

#### Seath

##### EF Member
Hello,
Would anyone be able to help with this question I do not seem to have the knowledge to answer please?

44
Questions 40 to 46 relate to the following scenario.
Insulation resistance has been tested on five new lighting circuits in an existing large
distribution warehouse. These circuits are supplied from a new single-phase, five-way DB.
Switching for each circuit is by 230 V contactors. Connection to each light is made using a
BS 1363 socket-outlet adjacent to the fitting.
The supply and installation form a 400 V three-phase TN-C-S system.
The following test results were recorded.

Circuit Line-line ohms Line-earth ohms
1 190 176
2 187 20
3 157 162
4 122 134
5 172 178

Initial and Periodic Inspection and Testing of Electrical Installations (2391-052)
Sample Questions
17What is the value of insulation resistance between Live and Earth for the new DB with all
the lighting circuits connected?

a)13 MΩ
b)20 MΩ
c)134 MΩ
d)178 MΩ

Response: correct answer is (a) I said (c)

#### SparkyChick

##### Making a banana smoothy for my fave gorilla
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Can you show how you worked out the answer?

I could give you the answer but that's not going to help much as it won't expand your understanding.

#### westward10

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#### telectrix

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if you look at the question. reading #2 shows 20. therefore the global resistance must be <20. (resistances in parallel.). therefore a. is correct without any working out.

#### westward10

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if you look at the question. reading #2 shows 20. therefore the global resistance must be <20. (resistances in parallel. therefore a. is correct without any working out.
No calculation required then, see I didn't spot that

#### telectrix

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No calculation required then, see I didn't spot that
specsavers late night opening is Friday.

#### Taylortwocities

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The ability to know about parallel resistances is a basic fundamental of electrical work..
I'm not knocking the OP, but surely this is first lesson stuff.

#### Andy78

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On a positive note, at least now you know what you don't know, and knowing that is better than knowing nothing.

Time to revise resistance in parallel.

#### Loki

Trainee Access
Rt = 1/r1 + 1/r2 + 1/r3 etc
Then the above is 1/r to give answer

#### Vortigern

##### Regular EF Member
The question states the value of IR results as ohms not Mohms would that make a difference to anyones answer? Must have forgotten to take the lamps out.

#### Wilko

Electrician's Arms
The question states the value of IR results as ohms not Mohms would that make a difference to anyones answer? Must have forgotten to take the lamps out.
I just took it for granted as rubbish wording and they meant megs

#### oracle

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The answer is easy but don't they give you marks for showing how you get the answer?

#### telectrix

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it's a tick box. no working out erqiuered,

reqiured.
required.
bloody beer.

#### westward10

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None of them are actually the correct answer, it is 13.34 what are these courses coming to.

#### oracle

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No working out? Bloody hell have they devalued the exam completely? The old 2391 written exam was a real bugger .

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