minor works cert help

[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?

 

[Octopus]

For overcurrent protection device think fuse or mcb

 

[Electricalserv]

Overcurrent protective device for that circuit is your MCB or fuse not main switch

 

[topquark]

Overcurrent device (if it's an MCB) will prob be BS EN 60898.

 

[sinaloan]

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

 

[topquark]

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).

 

[Electricalserv]

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

Depends what installation it is Domestic, Commercial etc

 

[sinaloan]

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?

 

[Electricalserv]

No but there is a maximium Ze. So if installation is over the maximum Ze (0.35 ohms on tnc-s) call the DNO

 

[telectrix]

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.

 

[topquark]

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).

 

[sinaloan]

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.

 

[topquark]

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.

 

[sinaloan]

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?

 

[telectrix]

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]

so the allowed pfc is up to 16.5ka?

 

[Goody]

...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.

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.

 

[topquark]

so the allowed pfc is up to 16.5ka?

It will depend on the rating of the main protective device in use (which is normally the DNO fuse).

 

[sinaloan]

so if the main fuse is 60a then my pfc could be?

 

[telectrix]

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.