Cut Outs & Isolators - Lucy Zodion Explain The Differences

By Chris Dodds on 7th September, 2015

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Cut Outs & Isolators - Lucy Zodion Explain The Differences

Cut Outs or Isolators - What’s The Difference?

Lucy Zodion


T&D invited Giles Davidson and Robert Ashworth from Lucy Zodion to guide us through the differences between using cut outs and isolators for roadway, tunnel, urban, industrial and street lighting applications.

Lucy Zodion is a leader in the design and manufacture of Street Lighting Equipment in the UK - this includes feeder pillars and appropriately cut outs and isolators.

Lucy Zodion can customise electrical distribution and control products to customer specifications as demonstrated by our recent record purchase order for Trojan 2 Isolators from Saudi Aramco.

Let's let Lucy Zodion educate and inform us on the key differences. 

"Confusingly, some people refer to cut outs as isolators and isolators as cut outs, that’s probably not what you want to hear, and to be honest as a manufacturer neither do we. Although the terms are treated as interchangeable there are clear distinctions between them. 

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.

They are installed behind the column door at the base of a lamp post (or street lighting column to give it its correct title), both comprising of an enclosure which is used to terminate a supply cable (typically armoured XLPE) and provide electrical protection and a means of isolating 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?

Cut Out

OK, a cut out has its own British Standard - BS7654, there’s a nice easy number to remember! BS7654 is the specification for single-phase street lighting cut-out assemblies for low-voltage public electricity distribution systems. 

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 25sqmm 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 and cable glanding 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 into 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 a qualified electrician.

Cut outs only come in either a single or twin fuse version with the option of using digital timers or RCCD’s to control or give additional protection aren’t available.

Cut outs only come in either a single or twin fuse version and it isn’t possible to use digital timers or RCCD’s to control or give additional protection. 

Picture : Lucy Zodion Titan Streetlighting Cut Outs

Lucy Zodion Titan Streetlighting Cut Outs


So how does an isolator differ?

Firstly the main body or enclosure of the isolator is not 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.

The isolator provides a “switch” which can be operated in order to isolate the load circuit, therefore compared with a cut out changing fuse(s) and isolating the load circuit is relatively simple.

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 use 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.

One thing that doesn’t come as standard with an isolator is a set of terminal blocks for the incoming cable (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.

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.

Note that the addition of extension troughs does of course add length to the assembly so it is important to ensure that sufficient room is available in the column.

Picture : Lucy Zodion Trojan Streetlighting Isolators

Lucy Zodion Trojan Streetlighting Isolators


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.

Unless there is a local authority specification or similar, in most cases the choice of cut-out or isolator will be determined by the type of installation and the functions required. 

If you know the size and type of incoming cable, whether it is looping in and out or not, if it is single or three-phase, plus the number of outgoing circuits required (often the number of lights on the column), then this is 95% of the information needed to work out what is needed. Beyond this the checks are mostly physical size etc.

Lucy Zodion and T&D are pleased to offer advice and assistance. 

For more information about Lucy Zodion and their range of isolators and cut-outs, click here.

Lucy Zodion


  • Further reading :

Lucy Zodion - Pre-Wired Power Distribution Feeder Pillars

Lucy Zodion Westminster Range of Retractable Feeder Pillars  

Lucy Zodion Dist RX Distribution Boards For Street Lighting Distribution  

Lucy Zodion Feeder Pillars Supplied For Perth Rail Station Power Project  


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Lucy Zodion Cut-Outs & Isolators - Catalogue

Category:  LV Cable Jointing & Terminating

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