Arc flash - Pulling the plug on unsafe practice


There are many things in life we do without thinking; closing the car door as we get in, buying a lottery ticket every week and, in many a factory, pulling plugs straight out of sockets! Now if it's the kettle plug we pull out before going home that's one thing. If, however, it's a three phase 415VAC equipment plug, then that can be much more risky explains Stephen Thackray of Marechal Electric

For one thing, the 13A kettle socket will be switched and most people would flick the switch first before withdrawing the plug. The industrial wall socket is commonly unswitched, or just with a separate rotary isolator fitted adjacent to the socket. This looks like an after-thought to protect the socket user by providing an isolator nearby and hoping it gets used. It depends on the isolator being operated before the plug is withdrawn from the socket to ensure the circuit is no longer live - hardly a fail-safe solution.

The consequence of pulling industrial plugs live out of sockets can simply result in the required disconnection of the electricity supply, but a much more serious outcome can be severe burning from electrical arcing or molten metal, usually brass - the metal widely used for plug and socket contacts. The fact that often nothing happens to the user when the plug is pulled out can lead to a false sense of security and risks the action becoming routine or entirely normal and, as such, one of those things we don't think about when we do do it. Things can become even more dire if there is a fault in the plugged-in equipment  - perhaps a locked rotor - then the danger of drawing an electric arc with the plug is extremely high. Preventing this outcome should be a corporate priority.

It's all in the design
The common industrial plug and socket design employs a brass pin fitting into a brass tube. Somewhere along this tube the male pin will make contact with the tube although not throughout the entire length of the tube - a forced fit.  The 16 and 32A versions are manageable, but the 63 and 125A versions test the strength and guile of the user.

Some socket versions offer an interlock to prevent live pull-outs but these don't always withstand the physical frustration of the user trying to disconnect the equipment. The common result of repeated plug insertions and withdrawals is contact wear and loosening. Loose contacts lead to overheating which leads to oxidization and eventual failure.

The abuse plugs and sockets seem to attract affects their serviceability. Conductor terminal screws loosen because nothing prevents them from loosening. The culprits are movement, thermal cycling and vibration. The cable entry point into the plug is often inadequate, forcing extra strain onto the conductor terminals themselves whilst the cable gland works loose and migrates up the cable sheath to perform no role whatsoever.

If there is one application where duty holders are sensitive to these failings, it is the transport industry - since refrigerated lorries must (increasingly) plug in overnight to cut down on noise pollution, engine wear and tear and fuel costs. It is common to see 415VAC 32A coupler sockets lying around on the tarmac suffering from the problems listed above but also quite possibly in or close to a pool of water. The RCD-protected socket posts will be at various angles having been reversed into by the trailer. The pin and sleeve design cannot offer a switched coupler socket thus trailers can be unplugged live with all the inherent dangers.

So what's the alternative?
The decontactor socket design approaches the contact problem head-on by using butt contacts instead of force fit types. The concept is it is safer to throw a switch before disconnecting industrial electrical plant and so the decontactor has an integral switch mechanism.  So far it's a plug and socket and a switch. But once the switch mechanism has been operated the isolator element comes into play. A decontactor is then a plug and socket, switch and isolator in one product - all the elements needed for safe electrical work practices.

This 3-in-1 design is made possible by the use of spring-loaded butt contacts. The silver-nickel tipped contacts mate head-on so there is no wear, no forcing of one into the other and disconnected by pressing a socket latch. Conductors are prevented from loosening by an anti-vibration terminal design. Pressing this latch is the only way to release the plug. This is particularly important if an extension lead is used. Holding the plug in one hand and the coupler socket in the other constitutes an ‘across the heart connection'. As the decontactor is a load-break device, the plug is already dead before it can be removed from its socket.

It's the simplicity of design that makes the decontactor such a versatile product giving the user the benefits of flexibility and above all safety on the shop floor.

Flexibility comes from the ability to have previously hard-wired equipment pre-plugged ready for easy re-location to suit production needs, from having equipment isolated quickly and safely by multiskilled staff and from the ability to take single phase supplies from three phase decontactor sockets. Motors fitted with decontactors can be replaced in a fraction of the usual replacement time.

User safety is provided ultimately by the electrical performance of the product. As an example, the 20A rated decontactor was successfully subjected to 10kA withstand and close-on tests - 80A delayed fuse, power factor 0.49 at 480VAC. Because the contacts close onto each other immediately, the current flows and, in the case of closing on a fault, trips the protection (ie. the fuse or circuit breaker).

Each socket contact has its own arc chamber and the socket is fitted with a safety shutter to prevent access to the contacts. As expected, sockets can be padlocked off to provide visible isolation. Most versions offer auxiliary contacts for controls or signals if required.

Reducing risks
Instilling safety procedures into the workforce is a fulltime job. Staff turnover means constant vigilance is required, particularly when the command of English is limited. Cleansing regimes, where required, remain unfriendly to electrical equipment particularly to sockets that are on the front line.

The decontactor, as a supply socket or motor disconnect, offers a much improved level of safety to users, backed up with positive test lab results, compliance to BS EN60309-1 (industrial sockets), BS EN60947-3 (air break switch) standards AC22, AC23 & AC3 and ultimately the Low Voltage Directive.