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The end of panel wiring as we know it?

10 November 2010

When building a control panel, wiring is almost always the most time consuming and costly operation. However, modern electronic technology is introducing innovations that could mean the days of conventional panel wiring are well and truly numbered. Phil George explains

Look inside a modern control panel and, depending on the application, you’ll typically find a programmable controller or a smart relay and a combination of motor starters, variable speed drives, soft starters, pushbuttons, indicator lamps and maybe even an HMI display. You’ll also find something else – a lot of control wiring to link all of these components together.

Now take a look at the plant or machine associated with the control panel. If you’d done this a decade or so ago, you would have seen even more wiring than in the control panel itself. Today, however, the amount of field wiring is minimal. The reason for this change is easy to understand. Once, every sensor, actuator or other field device was connected to the control panel with its own individual cable but now these cables have been replaced by fieldbus systems.

With fieldbus, multiple field devices can be connected to a single cable, so the number of cables between the control panel and the devices is dramatically reduced. Fieldbus technology of one kind or another has been in widespread use for well over a decade and produces big savings in plant installation time and costs. Yet inside control panels, ordinary wiring – directly comparable to the old-style field wiring – continues to be used. But why?

Part of the answer is that, because field wiring runs are long and costly to install, the potential savings associated with the elimination of conventional field wiring were greater than those associated with the elimination of conventional panel wiring. That made fieldbus systems attractive for outside-the-panel applications even when the technology was comparatively expensive and complicated to use.

But now things have changed. With the latest ASICs (application specific integrated circuits) the cost of producing fieldbus-type interface modules has fallen dramatically, and the powerful processing capabilities of the latest ASICs means that systems based on them can be made very easy to configure and use. In short, the time has come for the ‘fieldbus’, and all its benefits, to move inside the panel!

A first reaction might well be to think about adapting an existing fieldbus system for in-panel use. In practice, this is not a particularly good idea, as such systems necessarily have features that are optimised for field applications. Not only does this make them a poor match for in-panel use, it also means that the users invariably end up paying extra for features they’ll never need. A much better approach is a communication system that’s been developed from scratch with in-panel applications in mind.

Let’s have a look at the characteristics that would be most desirable in such a system. It must, of course, allow fast, simple and convenient interconnection of the components within the control panel. Daisy-chain connections are ideal, but some thought must be given to the physical form of these connections.

Ribbon cables provide an excellent solution, as they can readily be combined with insulation-displacement connectors. These connectors are placed over the cable at any required point and clamped in place using a special tool. The connectors then simply plug into panel components, such as motor starters, pushbuttons, and so on. Note that it isn’t necessary to cut, strip or otherwise prepare the cable.

In addition to being fast and easy, this method of connection has further key benefits – the connections can be made at any location on the cable to suit the spacing of the components; if a ratchet-type clamping tool is used it is virtually impossible to make a faulty connection; and extra connections can be added to the cable simply by clamping on another connector.

As well as the control signals to and from the connected devices, the ribbon cables can also be conveniently arranged to carry power, typically at 24Vdc, which the devices need in order to operate.

So much for the connecting method, but how will the communication system link with the panel components? One method is to produce specially enabled versions of the components and, with some products such as variable speed drives and HMI panels, this is a good approach. Another option is to offer standalone interface modules and, once again, these can be useful since they allow almost any type of component, however unusual, to be connected.

For motor starters in particular, there’s yet another option: why not simply have an interface module that clips on in the same way as an auxiliary contact block? This approach means that standard starters can be used, eliminating the need to stock different components for ordinary panels and those that use the new communications system.

Now, what about the links to the programmable controller (PLC)? It would be easy to make the system proprietary and limit its use to a particular brand of PLC, but a better option is to provide convenient interfaces that support a wide range of PLCs. And there’s another big benefit here – interfacing the in-panel communications system directly with the PLC eliminates the need for conventional I/O modules, leading to further substantial cost and space savings.

All of these benefits are of little value, however, if the communication system is complicated to use and difficult to set up. Fortunately, it doesn’t have to be. It is now possible to produce an in-panel communication system where setting up is limited to pressing a button and waiting for a few moments while the components set their own addresses. This done, the system is good to go!

As we noted earlier, fieldbus systems have replaced conventional field wiring in all but the simplest of modern applications, simply because they offer decisive benefits: they save money and time, they’re much easier to modify and they also aid the rapid location and diagnosis of faults. Now the same benefits are available for control wiring within panels, so what possible reason can there be for sticking to conventional wiring? Surely the end is nigh for panel wiring, as in-panel communication systems move in to replace it forever.

Phil George is with Eaton’s Electrical Sector

All of the features described in this article - and many more - are embodied in Eaton’s Moeller SmartWire-Darwin system. This not only provides a convenient and cost-effective alternative to conventional panel wiring, with wiring cost reductions of up to 60% easily possible, along with panel space savings of 40%, it also offers a wide range of advanced features.

For example, the latest PKE motor starters, when used with SmartWire-Darwin, can provide information about motor current via the network, thereby eliminating the need to use current transformers for monitoring. A SmartWire-Darwin system can also be extended outside of the control panel, to provide a complete internal and external ‘fieldbus’ system for simple applications. Alternatively, it can be linked to existing external fieldbus systems such as Profibus.

Another member of the SmartWire-Darwin product family is a software package that makes system design particularly straightforward. It also provides powerful facilities for examining the system status and, on the rare occasions they are needed, for diagnosing faults.

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