Technical Article

Wiring Today and Tomorrow

[picture of traditional wiring v SmartWire] Over the last decade or so, field wiring for automation installations has been completely transformed by the widespread adoption of fieldbus systems. Surprisingly, however, this revolution has had little effect on the wiring inside control panels. That's about to change, says Paul Bennett of Moeller Electric.

Few would deny that the fieldbus systems used in automation applications bring major benefits. Not only do they dramatically reduce the amount of field wiring required, they also virtually eliminate the risk of wiring errors and, possibly best of all, they make it easy to modify the installation if the configuration of the plant changes.

But open the control panel door, and inside you'll almost invariably find conventional control wiring, just like was once used in the field. This old-fashioned panel wiring shares all the disadvantages of old-fashioned field wiring. It's complicated, open to wiring errors and the only way to make changes is to re-wire the circuits involved.

Surely there has to be a better way – why can't the fieldbus benefits be enjoyed within the control panel? There's no fundamental problem with the technology. In fact, the same technology that's used for fieldbus systems would work just as well within the panel. But that's not to say that present-day fieldbus systems would be an ideal match for in-panel applications.

The first consideration is cost. With fieldbus systems used outside the control panel, the cost of the fieldbus hardware is invariably more than offset by the large savings made on cabling. Inside the control panel, however, the situation is very different – connections are short, so the reduction in cable cost is minimal. This means that the hardware for in-panel bus systems needs to be inexpensive if it is not to increase the cost of the panel unacceptably.

In addition, an in-panel system needs to be simple to use as panel builders, quite rightly, have neither the inclination nor the time to work with complicated configuration and addressing systems simply to link, for example, a few motor starters. Finally, the system must make economical use of space within the panel, given today's pressures for ever-smaller panels.

For these reasons, manufacturers of automation systems have, until now, seen little attraction in developing in-panel bus systems. However, progress in technology – in particular, the development of chips that can handle all of the requirements of an in-panel bus system efficiently and inexpensively – means that this situation has now changed.

In short, the time is now right for bus technology to come in from the field and move into the panel, so let's take a look at what an ideal in-panel bus system might look like.

[picture of traditional wiring v SmartWire] In order to minimise space requirements, add-on auxiliary blocks for contactors have long been designed so that they add little to the overall dimensions of the contactor. It makes good sense to adopt the same idea for in-panel bus adapters, designing them as compact units that simply clip into place.

But what of the connections themselves? It would be a retrograde step to partner modern bus technology with conventional screw-clamp connections. Plug-in wiring is a much better choice, particularly as it not only cuts wiring time dramatically, it also virtually eliminates the possibility of errors and badly made connections.

To satisfy the need for ease of use, the system should offer automatic addressing, and should also be capable of automatically reconfiguring itself when additional items are connected to the bus, or when existing items are removed.

Naturally, the in-panel bus must offer direct connection to PLCs, since this will enable significant cost savings to be achieved by eliminating the need for conventional PLC I/O modules. And, users shouldn't find themselves tied to PLCs from one specific manufacturer.

Open connectivity could be achieved by providing support for one or more of the popular automation communication protocols such as CANopen or Profibus. This approach offers the additional benefit of allowing the in-panel bus system to connect easily with conventional fieldbus installations. This is particularly convenient in distributed control applications, where some panels may not incorporate their own PLC.

As might be expected, all of the factors discussed have been taken into account in the development of Moeller Electric's new SmartWire system, which is already generating high levels of interest among panel builders and end users alike in mainland Europe.

We've mentioned some of the benefits that an in-panel bus-based connection system can bring but there are many others.

The time required to build the control panel is reduced, which makes it easier for panel builders to meet the tight deadlines invariably imposed by their customers. Panel builders can also start work earlier on the panels, even if requirements have not been completely finalised, safe in the knowledge that last minute alterations can easily be accommodated by changing a few plug-in connections. This is particularly true when the starters in the panel use a plug-in busbar system for power connections, as well as the in-panel bus system for control wiring.

Naturally, the time needed to test panels is also reduced, since the potential for wiring errors is minimal. Finally, maintenance throughout the life of the installation is made easier – if a starter is damaged, for example, it's only necessary to unplug the connections to it, clip a replacement in place, and replace the plug-in connections. No conventional wiring at all is needed.

Before we close, however, it's necessary to sound one note of caution. To maximise the benefits of an in-panel bus system, it's important to choose one that has been specifically designed with this application in mind. While it may be tempting to try to shoehorn a conventional fieldbus system into the panel, this is not ideal. The modules are, for example, likely to be too big, too expensive and, in all probability, too complicated.

When the appropriate technology is used, in-panel bus systems, as we've seen, have a lot to offer. In fact, it's very likely that, in a few years' time, conventionally wired control panels will be as rare as conventionally wired field installations are today. But there's really no need to wait – the technology and the benefits are available right now!

This page last updated: 15 November 2007