When to apply 1.2 factor to R1+R2...

junksamiad · Feb 25, 2019

I'm just trying to get my head round R1+R2 calculations (predicted rather than measured). All seems very straightforward naturally, however, I'm just after clarification of something. For the 1.2 factor that is applied to adjust the resistance of conductors to operating temperature, I just want to understand in what scenario I need to apply this?

For example (and to try and give a more specific context), if I am doing an initial verification on a circuit and I calculate my R1+R2, I know that by applying my 1.2 factor will provide me with the predicted resistance at operating temp. However, when I then measure this (under dead test conditions), I don't have to apply the 1.2 factor to my result before noting on the schedule of test results do I? I would be using rule of thumb figures for my max Zs permitted, I don't know whether this would dictate how I did things.

Thanks in advance for any help and clarification.

Strima · Feb 25, 2019

1.2 is used at the design stage to take into account increased conductor resistance due to operating temperature as you have mentioned.

When carrying out initial verification or subsequent testing such as a periodic it is much easier to multiply the maximum permitted Zs by 0.8.

It's the same thing just saves you doing the calculations on site.

ELECNEWT · Feb 25, 2019

Are there two factors: 0.8 for temperature effects and 0.95 for voltage variation? Giving final figure of 0.76?

Strima · Feb 25, 2019

The 0.95 is for Cmin. This is applied first to the max Zs but since the 3rd amendment of the 17th edition all max Zs published in the standard have Cmin applied.

If you refer to manufacturers information then you may need to apply Cmin especially for older devices that are still in use today.

If running from a private supply you may not need to apply Cmin.

Always apply the factors separately as both may not always be required.