Rotary Switch Utilisation Categories

It's a little more tricky than just switching on/off, as the actual category is defined in the relevant part of BS EN 60947 (or alternatively, BS 61095).

A (very long, approx 3 page) list of examples is provided in Guidance Note 2 Isolation and Switching (Table 3.2, page 39, in the 8th Edition, or page 41 in the 9th Edition)

Thanks. 

This is a bit of a minefield as without access to these product standards, it’s very difficult to fully understand the intent. 

The categories remain the same in any instance. The switch (and indeed something like a contactor) data sheets all indicate specific utilisation categories. I have seen standard installation contactors used on installs with attention only paid to the loading, rather than a specific category; these generally last for years (the coil is what usually fails). 

There must be a general rule that’s applied?

This 100A main switch had a UC of AC23: 

https://pdg.cef.co.uk/downloads/pdg/eaton_ems1001n_datasheet/eaton_ems1001n_datasheet.pdf

It makes no reference to resistive or capacitive load types, which would likely be connected as well as inductive loads, which is what AC23 relates to “Switching of motor loads or other highly inductive loads”. 

The isolator will be ON 24/7, therefore no mechanical switching will be taking place. I believe that generally the utilisation categories relate mainly to the switching on/off of the load, rather than the load itself during operation. 

I guess the thinking is that unless it's interlocked in some way, the main switch for an installation may well be used to switch on or off everything when under load - especially if it's called on for situations that require rapid (or emergency) disconnection or might be closed by ordinary persons who don't appreciate the advantages of switching off individual loads first.

   - Andy.

There is a list of common categories here. Now in general what happens if you select the wrong kind, is that the contacts burn up when you try and break an inductor under load (scope for high voltages needing rapid opening to suppress arc) or to connect to a capacitor (when there is a high current when the contact makes and the incoming voltage and the capacitor disagree.).

The trick to a long and happy life, as with many things electrical is to significantly under utilise the contacts, i.e. use a switch with a rating rather higher than the expected load.

In  practice the same contact may have more than one rating for different load types (classifications). The most benign sort of load  is resistive so this tends to have the higher figure.

For 'not sure'  AC-22 Switching of mixed resistive and inductive loads, including moderate overloads
is a good minimum starting point.

A23 (inductive loads ) is the one commonly applied to big gear like this (strictly an on-load switch disconnector)

https://www.tlc-direct.co.uk/Products/CMDFS100.html

So what is the maximum load on this installation. and are there known fixed loads ? 100 A perhaps.

and it sounds like it will operate rarely but can we know what the load will be at switch on/ off

Mike.

Hi Mike,

Thanks did your response. 

The load will be around 40A (it’s a small sub-main). The connected loads will be standard domestic type loading, such as an induction cooker, lights, immersion, electronics. 

The isolator will be installed at a mid-point between the main DB and the sub distribution, in theory left switched on forever. The switch is there for isolation only, and if we assume it’s turned off under load, this would happen circa 1-2 times per year max. 

Something equivalent to AC22 or AC 23 is probably about right for that sort of domestic load mix . Being a natural pessimist,  on a 40A fuse I'd aim for something nominally rated 63A or higher...
M.

Also, to be taken into account will be the cable size and type - 40A switches tend to have little room for bending cables into the terminals, so a size up is always a good thing, and gives a fighting chance of getting some stiff cable into position.