Given that an 'off load'  isolator should only actually be switched from on to off once the current has already fallen to zero due, presumably, to switching elsewhere, it is not unreasonable that  in some cases someone may want a lock to prevent it being turned off accidentally.
Though I cant think of a real example off hand...

Equally all sorts of switch gear is commonly locked to prevent operation at the wrong time by the wrong person, if need be by the use of locked doors on the switch room, so there is always that more general way of doing it.
Mike.

A designer is saying to me you can use an onload device (it s a rail application), but to me it clearly staes it has to be an off load device.

If you're thinking because you want the ability to lock off in the on position, it needs to be an off-load device, then i think you're reading it backwards. To me it reads, if you have a device that can only safely be opened when no current is flowing, then you should take precautions against it being opened when it shouldn't be. It doesn't read the same vice-versa.

   - Andy.

Hi Andy, the regs say

537.2.5   Provision shall be made for securing off-load isolating devices against unwanted or unintentional opening. This may be achieved, for example, by locating the device in a lockable space or lockable enclosure or by padlocking. Alternatively, the off-load device may be interlocked with a load-breaking one.

Its saying you can only lock on an off-load switch, AC-20 rating , the switch is in a public area hence the locking on, im totaly against it, I would just put on on-load isolator in a box.

Its in a public area, where you dont want anyone to switch it off, its only supplying a 110V AC LED lights.

personally I have never seen this done it just seems wrong to me. just put it in a box, then its compliant.

Alex Cooper said:

Its saying you can only lock on an off-load switch, AC-20 rating , the switch is in a public area hence the locking on, im totaly against it, I would just put on on-load isolator in a box.

No, Andy is correct ... where is the word 'only' used in Regulation 537.2.5.

The Regulation requires you to make sure that off-load isolators can't be switched off unintentionally (because there is a risk to a person who switches off an off-load isolator when current is flowing through it). You can achieve the requirement with a padlock, or locating the isolator in a suitable enclosure, for example.

This does not preclude other isolators from being locked 'on', and there are many applications in which this is done.

Provided anyone needing to use he isolator for safety reasons, such as maintenance, has a key to unlock the isolator, and uses another (unique) key to secure it in the off position when required, there is no problem. This is merely an operational issue for the owner/operator of the installation.

After all, there's not much difference between locking a larger-rated isolator "on" from using a security-switched fused connection unit or 20 A security switch?

If, on the other hand, a suitably-rated isolator were intended as an emergency switching device, it should not be locked in the "on" position, for obvious reasons ...

I can see it does not say “only”, but this is the only reference to locking on an isolator in the whole 624 page document. I can see it is open to interpretation, but it specifically “only” mentions an offload device. Which to me means only an offload device.

An offload device is not designed to switch current.

I need to add they they are proposing to use an on load isolater and padlock it on.

And what is the problem you see with that ? As described it sounds OK, presuming that the intention is the load stays on unless a suitable skilled/authorized person arrives with a key.

Mike

Alex Cooper said:

I need to add they they are proposing to use an on load isolater and padlock it on.

I understand that.

Alex Cooper said:

I need to add they they are proposing to use an on load isolater and padlock it on.

Here is an example, from the IET Electrician's Guide to Fire Detection and Fire Alarm Systems, 3rd Edition, that clearly shows that, not only is locking isolators in the 'on' position an accepted practice, but more importantly that it is actually required in some cases: