Hi, regulation 433.3 may help you. Please note amendment 4 will see regulation 433.3 updated - it appears you won't be able to omit overload protection as easily. 

This is correct, and not an uncommon situation, The cable needs to be OK for the largest credible load - limited by the breaker at the load end in this case, and also able to with stand whatever happens in a short circuit fault anywhere along it- which is a matter for the upstream fuse or breaker, and probably the adiabatic equation,

Cables supplying sockets are a bit of a grey area, as there is a small but not incredible risk of something else being plugged in by an unskilled person, that increases the load - perhaps the machine is changed for a bigger one at later date for example.

However, if the situation is one where such work is done by skilled persons, then its OK - if the load is one of many with that size plug and they are all on castors and move about, it probably would not be - this is more of an issue for 16A and 32A stuff that tends to be more mobile.

Note that sometimes larger plugs and sockets than are needed for the circuits intended load are fitted for reasons of standardization, or for loads where RCDs cannot be fitted.

But yes, it certainly can be OK, though it may need a bit of homework and or labelling to justify that approach,.

Mike

To clarify the load is only limited downstream by an OL unit, I understand the rational here, but how does this get around reg 433.1.1?

The cabling to the socket itself is fine, this is the manufacturers cabling from the plug to the kit

but how does this get around reg 433.1.1

Because the overload protective device in this case is the one at the machine - see 433.2 for the allowable positions of the protective devices (specifically 433.2.2).

Smaller flex than the upstream protective device is actually very common - most of the world uses unfused plus so 0.75mm² flex gets put on 16A or even 20A MCBs, even in the UK many appliances (e.g. fridges) have small flexes but need oversized (e.g. 13A fuses) to withstand starting currents. Another example is an unfused spur from domestic ring circuits - where the downstream plug fuse is in effect relied upon for overload protection of the spur cable.

   - Andy..

Ah well if this is the flex that came with the appliance, even a big appliance, then  it is not normally under BS7671 anyway, but a matter for the equipment maker's design/instructions.
Even so, the cable to the socket, or any 'extension lead' - and do I appreciate that a 3Pn at 125 is not a normal extension, but they do exist,  can also be undersized, so long as the load is limited by design, so long as the load characteristics mean that a serious  overload of the cable is incredibly unlikely.
Actually, as in the extension I have linked to, where such leads are needed, which tends to be for theatre and outdoor events with gensets, things actually tend to be oversized rather than under to allow for longer runs before voltage drop bites.
Mike.

mapj1 said:

Ah well if this is the flex that came with the appliance, even a big appliance, then  it is not normally under BS7671 anyway, but a matter for the equipment maker's design/instructions.

I agree ... BUT it's worth remembering that, especially with equipment in a workplace, the 'demarcation between standards' and 'reliance on manufacturer's instructions alone' will not necessarily protect an installer against prosecution, say under Section 6(3) of the Health & Safety at Work etc. Act, or EAWR (in this case perhaps Regulation 5).

So, I can see why the question has been asked.

Just wondering about the 125A MCB recommendation from the manufacturer. Seems very blunt with no type stipulation. I wouldn’t have thought a MCB was an appropriate choice for such a situation. MCCB with long time pick up adjustment or maybe MPCB or fuse might be better.

Having been recently involved in helping in the commissioning of a major installation in a milking parlour, I am not at all confident that manufacturers of some equipment are as technically aware as some might think. I don’t think it is necessarily prudent to blindly defer to their instructions. 

Hi Lyle, they actually suggest a "slow blow fuse", however the manufacturers panel includes an overload unit and circuit breaker, which, I guess negates the requirement for this

It may be that the innocent looking phrase "Slow blow fuse" tells a story. 

I suspect they are aware of a inrush problems that they are not entirely on top of ;-) Their fix is to specify an oversize supply, a thin and so slightly more resistive cable than normal, and to be a bit vague about speed ratings of fuses so its not their problem. If you have not already installed then be ready to change the breaker to an MCCB with a programmable delay or a maybe 'D' type MCB if there are inrush trip issues.

Or of course a slow blow fuse ;-)

Mike

The telling bit of information is how the term 'overload current' is defined.

From part 2: an overcurrent occurring in a circuit that is electrically sound.

In other words, by definition, if the characteristics of the circuit do not permit an overcurrent without a fault occurring then no overload protection is required at all. In this case as long as fault protection is afforded by the Zs being low enough to operate the protective device in time, and the conductors being large enough to pass the fault without damage, then the circuit is compliant. Incidentally, in this instance it's possible for the circuit to not meet I(z)>I(n)>I(b) yet still be compliant.