Calculating Fault Current...

[junksamiad]

Hi all,

quick question, hoping an easy one to answer. Just need to clarify this up in my own head.

When calculating fault current from Zs, do I take Cmin into account?

For example.

Zs is 0.54 ohms

So to find fault current I would calculate nominal voltage / Zs

So 230 / 0.54

But, should I be taking Cmin into account here and instead doing 218.5 / 0.54

Hope someone can take a minute to clear this up for me.

Thanks again in advance for any help.

 

[JAWS]

My understanding you are thinking about this formula Zs = (U0 x Cmin) / Ia where as the fault current calculation formula is I = Uo/Zs. So yours will be 230/0.54=425A

 

[junksamiad]

My understanding you are thinking about this formula Zs = (U0 x Cmin) / Ia where as the fault current calculation formula is I = Uo/Zs. So yours will be 230/0.54=425A

Right, thanks. So to clarify, you don't take into account any fluctuation in voltage when working out fault current?

 

[telectrix]

personally, i use the actual voltage as measured on site. this usually gives a higher fault current than using the nominal 230V.

 

[Baddegg]

Are you designing or testing existing?

 

[pc1966]

If you are checking for OCPD disconnection times then you would use the minimum supply voltage and Zs as you say (worst case for time to clear a fault).

If you are checking for the max PFC for any fuse/MCB rating of Icn / Ics then you would use the maximum voltage (worst case for breaking stuff).

 

[Strima]

When designing don't use Cmin, all other calculations are done using nominal voltage of 230 VAC such as voltage drop.

Cmin is only for adjustment of maximum permitted Zs for an OCPD to operate within prescribed times taking into account possible supply voltage fluctuations.

I=V/R

 

[pc1966]

personally, i use the actual voltage as measured on site. this usually gives a higher fault current than using the nominal 230V.

That is what the MFT reports - the measured voltage and measured Zs to give the PFC as tested.

 

[junksamiad]

That is what the MFT reports - the measured voltage and measured Zs to give the PFC as tested.

Thanks, that's brilliant. I'm just hypothsising at the moment but I can see now from the responses given why most of what has been said is logical.

I'm not an electrician, but I hope to start a career in it in the near future so I have been studying the math and theory for a couple of years in preparation for when I enrol and begin.

Therefore, a lot of my understanding is based on knowledge rather than skill, hence the apparent ludicrousness of some of my logic. But the above info is really helpful. Thanks again forum.

 

[Baddegg]

Thanks, that's brilliant. I'm just hypothsising at the moment but I can see now from the responses given why most of what has been said is logical.

I'm not an electrician, but I hope to start a career in it in the near future so I have been studying the math and theory for a couple of years in preparation for when I enrol and begin.

Therefore, a lot of my understanding is based on knowledge rather than skill, hence the apparent ludicrousness of some of my logic. But the above info is really helpful. Thanks again forum.

Good luck mate, you’ll be fine with that attitude ?