Custom and practice or a standard?

Jul 11, 2012 | Cables and Accessories

Guy Birchall, UK marketing manager, and Giovanni Baccini, European sales manager from Lewden Palazzoli, consider the major safety factors behind the growing trend towards switched interlocked sockets for many industrial and outdoor applications.

Although it may be tempting to dismiss certain electrical products as commodity items, one sector where this should not apply is industrial plugs, sockets and connectors.

Few people would argue that when it comes to specifying, installing or maintaining these products for industrial use, whether in a building, on a construction site, outdoors at an event or outdoor leisure location, an awareness of safety considerations is paramount, as well as a clear recognition of fitness for purpose.

On-site electrical equipment is at the receiving end of a lot of abuse, whether that is from the harsh environments which it has to endure or simply from misuse by hard pressed site personnel. Furthermore, the products used to connect this equipment to the electrical supply must function safely from the day they were installed into the far future. This point should certainly not be lost on those responsible and liable for safety, as the people ultimately using it could be members of the public.

Be aware of the risks

So what are the likely risks to a user when he or she is connecting or disconnecting a plug, or more seriously, if the appliance that requires the supply current is faulty? They are many, and varied, some well known, some not. There is always a possibility of exposure to direct or indirect accidental contact with live parts, giving rise to an electric shock risk, or the generation of unintended arcing which, combined with dust, might be explosive. None are welcome, and all can be avoided.

It is at the point of connecting or disconnecting a plug that a user is exposed to risk, known or otherwise. Prevention and protection against those risks is the circuit designer, component manufacturer and installer’s responsibility.

Lewden Palazzoli has jointly more than 150 years of experience in the design and manufacture of electric products, and it is the company’s firm belief that the separation and isolation of the supply from the plug and connector helps ensure an optimal solution. This requires a ‘switched interlock’ and although it’s a growing trend in the UK market, it is by no means common practice yet.

Circuit protection devices minimise the absolute risk, but only the separation of the electrical supply through an isolation mechanism can ensure the user is fully protected during connection and disconnection.

The engaging and disengaging of a plug into a socket, assuring the complete absence of power, can only be achieved by using a switched interlock socket. The separate supply isolator is mechanically linked or ‘interlocked’ within a single unit to the socket, so that a plug can only be inserted or removed once the supply is safely shut off.

However, care should be taken with respect to the specification of the isolator itself, which should ideally be rated at AC23/AC3, allowing the switching of inductive loads at full rated current (EN 60947-3).

It’s cheaper, contrary perhaps to popular understanding, than a standard socket. Customer feedback has highlighted that the speed of installation of a switched interlock is surprisingly quick compared to the installation of a separate isolator and socket. Also, do not forget that installing individual circuit protection in line with the 17th Edition becomes easier and cheaper still, as Lewden Palazzoli’s own evaluation and field testing demonstrates.

What’s available

Lewden Palazzoli offer a complete range of switched interlock sockets – 16-32-63-125A in thermoplastic, GRP and aluminium alloys. The company’s approach has been to exceed the minimum legislative requirements with regards to the critical importance of such product solutions. Lewden Palazzoli’s term for this approach is ‘Active Protection’ which embraces direct and indirect risk contact, fire hazard, environment (IP and IK ratings), and easy installation, cable access and maintenance.

All interlocks and individual parts should be made in compliance with the following international standards:

EN 60309-1: Industrial plug and sockets Part 1: general specifications.

EN 60309-2: Industrial plug and sockets Part 2: mechanical dimensions.

EN 60309-4: Industrial plug and sockets Part 4: switched sockets, with and without interlock.

EN 60947-3: Low voltage appliances Part 3: Switches, isolator switches, and fused units.

EN 60529: Enclosure protection rates (IP).


There is a strong case for the wide spread adoption of switched interlocks for all kinds of practical, legal, H&S and economic reasons. And, although Lewden Palazzoli will continue to stock the full ranges of plugs and connectors, the company would have no hesitation in recommending the switched interlocked approach as optimal.

Lewden Palazzoi /

T: 02085 390237