KMF or MCB to protect dist circuit

Using a KMF will provide better discrimination. Having 60898 circuit breakers in series will not provide discrimination. However a KMF may fall foul of regulation 421.1.201 and be classed as similar switch gear. So you would have to house the KMF in a non combustible enclosure which isn't ideal or use something like this Wylex switch fuse.

https://www.amazon.co.uk/Wylex-DSF100M-Amendment-Single-Cartridge/dp/B01E3X1QE8

out of interest what is the existing CU?

 

Go on then ? KMF been a spark for yrs dont know!!!

 

These switch fuses are generally referred to as a KMF. KMF was/is a part number for a MEM switch fuse like this but the name has kind of stuck for switch fuses of these types even though other manufacturers make ones that are very similar such as wylex, click etc.

 

Because depending on manufacturer, existing load, load of the new distribution circuit and a few other factors then you may of been able to use a fuse carrier in the existing board like this


https://www.bing.com/images/search?q=hager+fuse+carrier&FORM=HDRSC2

 

Another point is you will never be able to gland an SWA in to a KMF so you would have to use an adaptable box or something similar which IMO is a bit of a PITA.

 

In that case I would use the Wylex switch fuse I linked to earlier.

 

I would opt for the new Wylex switch fuse too. The new ones have a metal fuse cover....
You would have to gland your SWA into an enclosure before the KMF, so a metal clad switch fuse is the way to go.

I tend to use the gland set TRS gland for the tails, even though not particularly required, and intended for TT installs. Keeps things neat.

 

I've been using these

http://remora.net/catalogue/nylon-c...ail-Kits/GripLoc-Consumer-Unit-Tail-Kit-Nylon

I've been getting them from KEW electrical supplies for about 10pence more than what's advertised on remoras site but don't have to pay the postage. Cheaper than Wiskas equivalent.

 

TBH, I have no idea how much I pay for them, will look next week

 

This situation has nothing to do with discrimination between disconnection devices. The OP is a muddled question, tbh. A KMF is a fused switch which simply has a fuse in the phase side. There is no reason you could not adopt a double pole mcb arrangement if you wanted to or take a new circuit off the existing CU if this was practical. A 60A or 80A BS1361 will not discriminate with the main cut-out. But the question you need to ask yourself is are you really that concerned that the load from a shed would come close to overloading a 16mm2 SWA? There is really no need to put any fused device on the front end as the main cut-out will be there exactly as it would if you were coming directly off the CU.

 

Lee means discrimination between downstream MCBs and the protecting MCB. During a fault of high fault current, you cannot guarantee which MCB would operate, it could be the 32amp in the subboard or the 63 protecting the submain. With a fuse, you have discrimination between the submain and subboard.

It is not uncommon to see a fault a on a small sub circuit take out an MCB protecting a submain.

We don't know what is in the shed, but we must assume it's more than just a shed if 16mm and a KMF were to be used.

 

Absolutely spot on, my thoughts exactly.

 

No it doesn't. That's the point of my reply. All the fuses will be in the shed end protecting the SWA. What is the point in putting a device between the SWA and the cut-out? No discrimination is available and it is unnecessary.