low psc reading on tt system

[cs97]

Hi

Recently come across a submain installed in a property that had a 63 amp d type mcb and a 100ma s type rcd as its overcurrant and earth protection. Fair enough. I measured a psc of 558 amps. Voltage 248 amps Ze 128 ohms. At origin. As an mcb is designed to operate in .1 of a second would this low psc reading affect its characteristics?

Many thanks

 

[yellowvanman]

RCD will deal with any earth faults.

For a short between L-N you would get 558A - looking at the characteristics of a 63A D Type 60898 (Page 251 of 17th) the disconnection time is somewhere between 5-6 seconds.

 

[cs97]

Ok cheers for ya help mate. Good man. I was thinking of swapping it out to a b type. I found that the supply authority 60 amp fuse would need 330 amps to blow in 5 seconds. Thanks tho mate.

 

[Deleted member 9648]

Regulation 435.1 is applicable here.

 

[cs97]

Ok il have a quick look Cheers

 

[cs97]

Thanks for help fellas,

Under reg 431.5 are we to assume that the supply authorities main fuse is to disconnect in less than .1s ? This measurement would not include the added resistance of the 3 m of 25 mm tails from the meter to. Bit of a mine field