minimum insulation resistance Mohms

[swifters]

Hi, I'm studying for my 2391 and tlc website gives the table below. 17th regs book gives 1 MegaOhm for 500v so I take it TLC are basing this on older regs book? Thanks Jon

http://www.bathroom-london.co.uk

Extra-low voltage circuits supplied from a safety transformer

250​

0.25​

Up to 500 V except for above

500​

0.5​

Above 500 V up to 1000 V

1000​

1.0​

 

[IQ Electrical]

Yes, stick with BS7671:2008 and Guidance Note 3, also it's 1 MegOhm, not MegaOhm (terminology is key in the 2391-10 exam)

 

[swifters]

In my copy of Guidance note 3 it agrees with TLC but I suppose I must have an out of date copy?

 

[IQ Electrical]

What issue are you using?

The current edition is issue 5.

 

[swifters]

Not sure, it says on front including amd no1:2002. Perhaps not the most recent? Doh. I didn't realise there were diffrent versions of GN3.

 

[IQ Electrical]

You're off to a bad start, a 9 year old Guidance Note 3 and a web site using 16th edition IR values!

I have to say I'm surprised you didn't pick up on the different values sooner, there have been quite a few changes (disconnection times, max Zs values, volt drop bands, IR values etc.)

I take it you haven't got a 17th Edition qualification?

 

[swifters]

No, I have the 17th. Did they change from 250v 0.25Mohms to 0.5Mohms and 500v 0.5Mohms to 1Mohms for the 17th?

 

[swifters]

RCD trip times are still 40ms for 5x and 300ms for 1x i hope?

 

[ian.settle1]

No, I have the 17th. Did they change from 250v 0.25Mohms to 0.5Mohms and 500v 0.5Mohms to 1Mohms for the 17th?

Went back to the former values that where in earlier regs editions

 

[ian.settle1]

RCD trip times are still 40ms for 5x and 300ms for 1x i hope?

Depeds upon whether it is a BS or BSEN number on the RCD for a BS it was 200ms and 300 ms for a BSEN.

 

[telectrix]

am i right in assuming its 20mS and 200mS for TT systems?

 

[IQ Electrical]

RCD trip times are still 40ms for 5x and 300ms for 1x i hope?

Yes, 40ms at X5, 300ms for X1 (for 61008/61009)

For a BS4293 device, 1X is 200ms.

Obviously selective time delayed devices are different.

 

[swifters]

Thanks, that all makes sense. It was a bsen. Will have to remember 200ms for bs. Why for a tt system is it 20ms and 200ms? Dont understand that one.

 

[IQ Electrical]

am i right in assuming its 20mS and 200mS for TT systems?

Yes but remember that those TT disconnection times are for a 'fault of negligible impedance'.

 

[swifters]

Yes but remember that those TT disconnection times are for a 'fault of negligible impedance'.

Just one last question, why do you have to multiply the highest line to neutral value by 2 to obtain 3 phase prospective short circuit current?