Ze for design calculations

[JameZZZ]

I have a question for the designers out there.

when carrying out design calculations prior to a new supply being installed what value of Ze do you use?

ON the limited number of occasions I have done this myself I have always used the highest allowable (eg 0.35 for TNCS or 0.8 for TNS) when estimating Zs for disconnection times and a suitable low value of Ze when estimating PFC. I have always done this as a worst of both worlds. This normally means I have to use electronic MCCB's instead of Thermal Magnetic ones because I cannot get the trip times otherwise.

I only ask this because I have had a conversation with a pal of mine and he says he uses much lower values of Ze and therefore gets much lower Zs values. This way he can design in cheaper MCCB's with less sensitive trip characteristics.

Who is correct, what is the industry standard for this?


Thanks in advance for your replies.

JameZZZ

 

[Octopus]

I would say using the "top" value is the way to do it!

Cheaper MCCB's - not exactly the way to do decent designs!

 

[Plonker 3]

As Murdoch said. Use the higher value then when on sitr you know your Zs will be compliant. What does your friend do if his onsite Zs value is higher than what he calculated?

 

[JameZZZ]

Dillb,

I am not sure what he does? I suppose he crosses his fingers during the Initial Verification

 

[davesparks]

Or just makes up the results on his test sheets