High PFC

[ohthatshowyoudo]

Hi all
i have a question regarding a high pfc and installed mcbs. can i rather than replacing every mcb incorparate a main breaker equal to the pfc ?

 

[Guest123]

Can you give us the scenario in a bit more detail.....magnitude of PFC, type of premesis, ratings of switchgear etc etc.

 

[johnboy6083]

i think its called cascading. yes you can, and its often done in industrial jobs, and on certain domestic jobs where the DNO fuse is used.
the problem is ensuring that all of the electrical gear upto that point can take the fault current without bursting into flames, or causing serious damage.

 

[ohthatshowyoudo]

yes the pfc is 11ka on l-n and 5 ka l -e the current devices are rated at 6 ka so rather than replace all mcbs i wanted to incorparate main breaker at 15 ka does this sound acceptable ?

 

[johnboy6083]

it all depends if your DB is rated for the fault current. i cant remember the appropriate standard, but im sure that someone will be along shortly to let you know.

 

[Guest123]

The reason I ask is that even though the MCB's are only 6KA if they are in a consumer unit To BS 60439 and protected by a BS 1361 type 2 suppliers fuse then the whole assembly is tested upto 16KA.

Here is a snippet of annex ZA......

In the UK, such equipment is referred to as a ‘Consumer Unit’
and as such is covered by this Standard. However, additional
requirements from annex ZA of the standard call for the
assembly to have an additional test; this is known as the
‘conditional withstand test’. The condition is that the consumer
unit must withstand a 16kA short circuit fault when protected by
a 100A HRC fuse to BS 1361 type II. Further requirements are
addressed by this annex:
• Means of isolation to be via a manual double pole switch
disconnector
• Rated current of consumer unit is determined by current
rating of incoming device.
• Outgoing protective devices can be MCB’s, Fuses and/or
RCBO’s.
* No diversity factors are applicable to consumer units, the
incoming circuit and the bus-bar system must be able to carry
their full rated current without exceeding the temperature rise
limits.