minor works cert help

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Discuss minor works cert help in the Australia area at ElectriciansForums.net

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sinaloan

hi there i am filling in a minor works cert and in part 2 it asks for 'overcurrent protective device for the modified circuit' and 'residual current device'. i have a split load board with one rcd on it. the work has been carried out on a ring circuit on the rcd side so i assume i put in the rcd box i fill in the rcds details? in the 'overcurrent protective device for the modified circuit' box do i put in the mains switch bs number 60947-3 or do i put in the main fuse bs number 1361?
 
ahhhh ok now i see thanks very much guys. also a quick one i have changed a fuseboard and im filling in a domestic electrical installation certificate. when would the next inspection date be due? is it ten years? this is for my nic eic domestic installer application
 
Default recommendation is 10 years or change of occupancy for domestic (unless usage/age/condition would dictate otherwise, ie older wiring etc with low IR).
 
sinaloan said:
ahhhh ok now i see thanks very much guys. also a quick one i have changed a fuseboard and im filling in a domestic electrical installation certificate. when would the next inspection date be due? is it ten years? this is for my nic eic domestic installer application
Click to expand...

Depends what installation it is Domestic, Commercial etc
 
ok thats great sorry electricalserv i meant to say its domestic so ten years it is. also is there a maximum pfc thats allowed? its a tncs system?
 
what is your Ze and pfc as measured? don't forget . you measure Ze with main earth dissed and pfc with earths and bonds connected.
 
Maximum is normally dictated by the DNO's fuse max rating. I believe that the minimum recommended for them is around 16.5 KA Most main switches 60947-3 are rated up to 6 KA (and it should be marked on each BS standard compliant device).
 
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yes thats great i understand the ze, zs, ir, cont tests etc and know why you do them and what all of the expected results should be and why but iv never understood pfc and what its about and what the reading should be or mean. my college were rubbish and i learnt more over the three years researching myself.
 
When measuring pfc you are attempting to ascertain what is the largest current your equipment could reasonably be expected to handle safely. In most domestic installations you are generally relying on the DNO main fuse for that (whatever type it is). You may want to "beef up" your CU if you get anything over 6 KA. Remember it may be a much higher potential current (and a probable problem) if you have a really good earth ie < 0.0139 Ohms.
 
telectrix my measured ze was .20 and my pfc was 1.25ka this installation is a tns so the ze measurement is ok. is the pfc? and how am i meant to know if its ok or not on other installations?
 
basically your pefc is the current you would get if there was a fault of negligible impedance ( dead short ) between L and E. using ohms law you woud expect I = V/R, or V /Ze. i.e. 230/0.35 ( taking the max. Ze for TNC-S ). = 657A . obviously, the values of V and Ze will vary.
 
sinaloan said:
...but iv never understood pfc and what its about and what the reading should be or mean. my college were rubbish and i learnt more over the three years researching myself.
Click to expand...
The prospective short circuit current (PSCC) is the highest electric current which can exist in a particular electrical system under short circuit conditions. This value dictates the type (capacity) of breakers used in the circuit.

When a large electric current is interrupted an arc may form and if the breaking capacity of a fuse or circuit breaker is exceeded then it may not be able to extinguish this arc meaning that the current will continue, possibly resulting in a fire or explosion.
 
no. the pfc should not exceed the breaking capacity of the fuse, not it's rating. the DNO fuse capacity is usually 16.5kA, but it deoends on the type of fuse.
 
Have a read of this snippet BS 60439, Annexe Za.....


"Part 3 of the Standard, BS EN 60439-3, deals with the particular
requirements for low voltage switchgear and control gear
assemblies intended to be installed in places where unskilled
persons have access to their use. This covers the supplementary
requirements for enclosure distribution boards suitable for indoor
use containing protective devices intended for use either in
domestic applications or other locations where unskilled persons
have access. Control and/or signalling devices may also be
included.
These distribution assemblies are for use on AC supplies, with a
nominal voltage to earth not exceeding 300V. The outgoing
circuits include short circuit protective devices, each having a
rated current not exceeding 125A with a total incoming load
current not exceeding 250A.
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."
 
Last edited by a moderator:
topquark said:
Maximum is normally dictated by the DNO's fuse max rating. I believe that the maximum recommended for them is around 16.5 KA Most main switches 60947-3 are rated up to 6 KA (and it should be marked on each BS standard compliant device).
Click to expand...

Ooops, that should be minimum!! I'd best fix that :eek:
 
sinaloan said:
so if my pfc was over 6ka topquark then id have to upgrade them as this would exceed the breakers ssc?
Click to expand...

No, as you are protected by the DNO's fuse breaking capacity, assuming that is large enough. As stated they must have a minimum of 16 KA for a 230V system (I think it's 33 KA for 3 phase).

You should not rely soley on the DNO fuse for overcurrent though, if that's what you are thinking, they don't very often fail that quickly at rated current capacity (an 80A fuse may supply current at 100A for some considerable time) and you wouldn't want to break the seal to replace it after testing would you now ;)
 
ok cool so the best way to work out my maximum demand is to use the diversity factors in appendix 1 of osg. does everyone do it like this or is there an easier way to work it out, someone told me to just put down the rated current of the main fuse but thats wrong right?
 
I think this thread is getting to be like that pantomime song

"There's a hole in my bucket dear Lisa, dear Lisa"

and finishes

"There's a hole in my bucket dear Lisa, dear Lisa a hole" :)
 
when i do the testing on my fuseboard i take it i cant pull the main fuse then? do i just turn the mains switch off to turn it off and start testing or take the tails out of the meter? how fussy will the nic inspector be
 
Just putting the rated current of the main fuse is wrong (well assuming the circuits load was designed correctly ;)). There are some "rules of thumb" that are often used. I'd suggest going though the exercise of doing it by the OSG though, coz it's useful to have that off the top of your head as well.
 
sinaloan said:
when i do the testing on my fuseboard i take it i cant pull the main fuse then? do i just turn the mains switch off to turn it off and start testing or take the tails out of the meter? how fussy will the nic inspector be
Click to expand...

No, you can't pull the main fuse (it'll be sealed anyway). You turn the main switch off then perform a safe isolation to prove that your circuits are dead.
 
Putting them in rubber gloves (use separate finger holes) is lot safer, but make sure you have checked the rubber gloves rating first, LOL!
 
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haha guess what everyone i passsed!!!!!!!!!!!!!!!!!!!!!! wahooooooooooooo. im now a domestic installer! thanks for all the help i really appreciate it. it was bloody simple in the end.
 

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