Advice on how to calculate these 2 Questions

[Bear]

Afternoon all,

How do i solve these two questions, Feel like am missing something simple. doing my head in lol.

I need to learn the calculation for them both, I am struggling to work it out.

1.
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.
What 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


2.
Continuity of a ring final circuit has been tested as part of an initial verification of a new primary school. All the socket-outlets are connected directly to the ring.
The circuit loop length is 58 m long, wired in 4 mm2 live and 1.5 mm2 cpc flat profile cable. The circuit is protected by a 32 A BS EN 60898 Type B circuit breaker.
What is the expected measured value at each socket-outlet when the line and neutral conductors are correctly cross-connected?
a) 0.13 Ω
b) 0.16 Ω
c) 0.22 Ω
d) 0.49 Ω

 

[ChipFanDave]

If you are a qualified Electrical Engineer why are you on here asking ?

 

[James]

You need to brush up on ohms law and
resistors in series and parallel calculations

If you are in training, there is a complete section of the forum, dedicated to people learning the trade.

I recommend you apply to join it.

 

[Zerax]

As already mentioned... resistors in parallel, Page 148 of GN3 and join the training forum.

 

[Wilko]

Hi Bear - for Q1 recall that putting resistors in parallel reduces the total and the smallest value here is 20 meg, so it must be less than that with all the circuits included.

 

[Pete999]

Afternoon all,

How do i solve these two questions, Feel like am missing something simple. doing my head in lol.

I need to learn the calculation for them both, I am struggling to work it out.

1.
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.
What is the value of insulation resistance between Live and Earth for the new DB with all the lighting circuits connected?
View attachment 61232

a) 13 MΩ
b) 20 MΩ
c) 134 MΩ
d) 178 MΩ


2.
Continuity of a ring final circuit has been tested as part of an initial verification of a new primary school. All the socket-outlets are connected directly to the ring.
The circuit loop length is 58 m long, wired in 4 mm2 live and 1.5 mm2 cpc flat profile cable. The circuit is protected by a 32 A BS EN 60898 Type B circuit breaker.
What is the expected measured value at each socket-outlet when the line and neutral conductors are correctly cross-connected?
a) 0.13 Ω
b) 0.16 Ω
c) 0.22 Ω
d) 0.49 Ω

You should be able to tell us how to calculate this this you're the Electrical engineer

 

[telectrix]

2. From tables in BS7671, mOhms /m for 4 0mm and 1.5mm, calculate the values for 58m of each.
( R1 and R2), add the 2 together . divide by ? work it out.

 

[H]

2391 by any chance?

 

[Bear]

Wow!! Never had a response like that before, Took me a day to find out what you all where talking about?

I am doing my 2391 - 52, I was a sparkie for 14 years but the credit crunch hit in 2010 so changed Jobs, now am back after almost 10 to 11 years and am struggling with it all.

 

[DPG]

Have you done the parallel resistance sum yet? What do you get?

 

[telectrix]

post #5 sez you don't need to do any calculations. think about it.

 

[DPG]

post #5 sez you don't need to do any calculations. think about it.

Deleted. Apologies ,
[automerge]1602068769[/automerge]

post #5 sez you don't need to do any calculations. think about it.

No, but he needs to know how to do it.

 

[telectrix]

Deleted. Apologies ,
[automerge]1602068769[/automerge]


No, but he needs to know how to do it.

why. it has to be <20Meg, so answer a) is the only possibility.

 

[DPG]

why. it has to be <20Meg, so answer a) is the only possibility.

My fault, I could have sworn there were two answers less than the lowest figure. Must have read it wrong. He still needs to know how to calculate parallel resistances though.

 

[telectrix]

for Q2 i get answer d.

 

[DPG]

for Q2 i get answer d.

Haven't done that one yet.

 

[Zerax]

for Q2 i get answer d.

Can you show your workings on that one Tel ?

 

[DPG]

Can you show your workings on that one Tel ?

Not until the OP has had a go though!

 

[JAWS]

for Q2 i get answer d.

.
Q2 I get A

I have probably done it wrong but I went 58 x 4.61 = 267.38/1000 = 0.26738/2 = 0.13369

 

[Zerax]

Not until the OP has had a go though!

Good point ! Come on Bear... we've all got work to do here...

 

[telectrix]

just re-read q. it'sL and N cross connected. i did L and E... DOH.

 

[H]

Have you got a scientific calculator..? How do you ‘tag’ people on here? ?

 

[DPG]

Have you got a scientific calculator..? How do you ‘tag’ people on here? ?

Just download a decent calculator app to your phone.

 

[telectrix]

.
Q2 I get A

I have probably done it wrong but I went 58 x 4.61 = 267.38/1000 = 0.26738/2 = 0.13369

agree. just done it again.

 

[Bear]

Hi sorry, got major pc issues, (GPU faulty)

Yes i have worked out both questions now sorry for late reply,
Q1 GN3 page 51,
Q2 i think have worked out as 4.61/2 x 58=133.69/1000 = 0.133.
So answer is A 0.13 Ohms

I am not sure if that Q2 is the correct way but that's what I've come up with in my hunting around old collage stuff. Unfortunately my writing was just as bad 20 years ago as it is now lol...