Earth Fault Impedence

[Loyal]

Can anyone please explain what is the maximum earth fault loop impedance permitted for moulded case circuit breaker (MCCB)?

Also, BS 7671 says a disconnection time of 5 sec is allowed for breakers in distribution circuits. What is the difference between a distribution circuit and final circuit?

 

[sellers]

A distribution circuit feeds another distribution board/consumer unit

A final circuit serves a load, such as lighting socket outlets, shower.

5 sec max is for distribution circuits and final circuits over 32amps.

0.4 is for all other circuits. When using BS60898 MCBS 0.4 and 5sec max Zs (Earth fault loop impedence) values are the same.

The max value depends on the breakers Size and Charactoristics.

All values can be found in bs7671, big red book.

 

[Andy™]

you will have to get max Zs values for MCCB's from the manufacturer - try their website

 

[sellers]

ahhh, MCCB's the bigger ones, my bad

 

[kme]

ahhh, MCCB's the bigger ones, my bad

One of them......

n.b. Anybody see anything wrong?????

 

[sellers]

Well some of the ones i've come accross have had extra plastic shrouds over the incoming cables, but those eaton ones look well insulated.

Could be the blank links? Not in place or cut off?

 

[kme]

Blanks`re fine - shrouding busbar terminals (if you want to be pedantic, they didn`t clip `em together.......).

Next??

 

[sellers]

I was clutching at straws KME, but just spotted 16mm on a 100A MCCB?

 

[kme]

Welllllllllll....

Suppose I tell you that the EIC for the install is telling me that the three MCCBs on the left are ALL connected to 25mm XLPE/SWA?

 

[septiclecky]

Welllllllllll....Suppose I tell you that the EIC for the install is telling me that the three MCCBs on the left are ALL connected to 25mm XLPE/SWA?

How the flip were we to know that we're not mind readers:C

 

[kme]

Nooooooo, but you would still question a 16mm on 100A protective device; especially when the cable size doesn`t match the schedule!!!!!!

 

[sellers]

So wheres my scoob?

 

[DrVoltage]

Interesting stuff guys

 

[kme]

So wheres my scoob?

Awarded

 

[Loyal]

Thanks all for good information. I have one basic question. What is the purpose of measuring earth fault loop impedance? It makes sense to measure earth fault loop impedance if the protective device is earth leakage breaker or RCD; but why measure the earth fault loop impedance for regular circuit breakers?

 

[kme]

To verify that enough current will flow to "trip" the protective device within the necessary maximum time for that circuit

KME