Confusingly some people call cut outs isolators and isolators cut outs, that's probably not what you want to hear, and to be honest as a manufacturer neither do we.
Let's get back to basics, in essence both products set out to do the same thing, they are both primarily used in the street lighting market and they sit behind the door at the base of a lamppost (or column to give it it's correct title), both comprise of an enclosure which is to terminate a supply cable and provide electrical protection for the light on the column.
If it's the installers choice on what to install, both products would be suitable, however the choice isn't always down to the installer, so they should consult with their client to see if they have a specification they need to comply with.
So, if both products do the same thing, what's the difference between a cut out and an isolator?
OK, a cut out has its own British Standard "BS7654, there's a nice easy number to remember! This BS covers all aspects of the cut out from the materials it's manufactured from to a series of tests relating to temperature, ingress protection, current, mechanical strength; it even states its physical size. You should ask if the manufacturer has independent certification, good ones will and should be happy to send you a copy.
The cut out will come with a set of terminal blocks designed to accept cables up to 25mm2, and will have a suite of accessories that include extension pieces and brass and plastic cable entry plates to suit different types of cable and to make installation as easy as possible.
The fuse in the cut out is contained within the cover of the unit, the action of removing this cover disconnects the fuse (so it can be replaced) which also isolates the load from the supply. Better cut out designs (such as Lucy Zodion of course) have a lever cam action handle on the cover which saves greatly on grazed knuckles or falling backwards in to the road when removing the cover. It's worth pointing out that although the terminals are isolated removing a lid from a cut-out to change the fuse should only be performed by an electrician.
Cut outs only come in either a single or twin fuse version and the option of using digital timers or RCCD's to control or give additional protection aren't available.
So how does an isolator differ?
Firstly the main body or enclosure of the product isn't covered by a BS, however reputable manufacturers will still produce the body in the same material as the cut out, and again independent certification should be available. Factors like a high tracking index and self-extinguishing plastics are vitally important when it comes to safety.
The isolator is in essence an enclosure with a DIN rail that typically can accept products like MCB's and fuse carriers which are in total no more than four modules in width (a module is a product 17.5mm in width). Think of an isolator as a mini distribution board if you will.
Typically the isolator comes with a 32 amp isolator, hence the name, and a fuse carrier, however as the product has a DIN rail different arrangements and components can be used e.g. digital timers, MCB's, RCBO's, RCCB's, contactors, push buttons etc. An isolator can form the basis of a unit used for Festive Decorations for example.
The isolator range will also have a suite of accessories that include extension pieces and brass and plastic cable entry plates to suit different types of cable and to make installation as easy as possible, but the one thing that doesn't come as standard is a set of terminal blocks unlike the cut out; so the installer needs to take this into serious consideration and add in terminal blocks to the final assembly to ensure installation is possible, this does in some instances make it quite long in length.
The isolator provides a "switch" which can be operated in order to isolate the load and/or for the fuse to be replaced, and therefore compared to the cut out changing fuses and isolating circuits is relatively simple.
So in summary, the cut out is a great work horse providing a method of terminating cables in straightforward situations where no more than two load circuits are needing protection. The isolator can offer greater flexibility of protection and control but can be more complex in some scenarios.