Technical Article

Expect More from Your Breakers!

[picture of NZM units] Gone are the days when the only task of a moulded-case circuit breaker (MCCB) was circuit protection, says John Towersey, Technical Director of Moeller Electric Limited. Today's products are also powerful tools for effective plant monitoring and maintenance, and building services.

MCCBs, like all circuit breakers, continuously monitor the current in the circuit they are protecting. Information about the instantaneous value of that current and how it has varied over time, particularly in the period immediately leading up to a breaker trip, is invaluable as a guide to system performance, and as an aid to the speedy location and rectification of faults.

Unfortunately, with ordinary MCCBs, the information simply isn't available, unless costly external measuring and monitoring equipment is installed. This situation is changing, however, with the spread of MCCBs which have electronic trip systems. In addition to their basic protection function, these trip systems can be designed to do something that no conventional thermal-magnetic trip ever could – capture and store data.

No longer is an engineer investigating a fault confronted with a tripped breaker but very little guidance on why it has tripped. Instead, the information needed to diagnose and repair the fault is readily to hand. Even better, users can be alerted to impending trip conditions, so that action can be taken to prevent an interruption to electrical supply which will always be inconvenient, often costly and sometimes, as in hospitals, potentially life threatening.

In order to take full advantage of these benefits, what should specifiers and users look for in a data-equipped MCCB? Let's take for granted protection performance, on the basis that modern products from reputable suppliers can be relied upon to fully meet the requirements of the exacting international standards now in place. Let's instead look at the additional functionality.

As we shall see, facilities for extracting detailed information from an MCCB are invaluable, but equally important is the option to assess its basic operating status at a glance. This option can readily be provided by an LED on the MCCB which indicates when the load reaches, say, 70%, 100% and 120% of the trip value.

This is a useful first level of communication, but there are many instances when more detail is needed than a simple LED can provide. A convenient solution is a plug-in terminal with a display large enough to show key parameters, and a keypad to allow navigation through the various items of data that are available. These terminals are often small hand-held devices that can be moved from MCCB to MCCB as needed. In some applications, however, a permanently connected display may be preferred, so the option to mount the terminal on, for example, an enclosure door, is desirable.

[picture of computer linked to MCCB units] An increasingly popular alternative is to use a laptop PC, running software provided by the MCCB manufacturer, to inspect and extract data. The benefits are that most users will already have a suitable PC, and that the data captured can be easily exported to standard office programs for further analysis or for inclusion in reports.

Having facilities for accessing and analysing data from the MCCBs is all very well, but what sort of data should be available? Real-time current information is a basic necessity as is, in the event of a trip, a clear indication of the trip type – overcurrent, shortcircuit, earth fault, etc. Additionally, historical information is invaluable and should include a time- and date-stamped log of events such as trips and trip warnings.

Details of the way in which the currents in the circuit changed in the run up to a trip are a vital diagnostic aid, and this information is usually provided by recording current data on a revolving basis. This means that the MCCB has capacity to store data covering a fixed period of time and that, when the store is full, it over-writes the oldest data first. Recording stops when a trip occurs, allowing data for the period immediately prior to the trip to be accessed. Given the present-day low cost of data storage, manufacturers may also decide to include other information, such as device identification data and even documentation.

In many modern applications, MCCBs are used alongside networked industrial or building automation systems, and it is highly desirable that they can exchange information directly with these systems. Interfaces for popular networking standards such as Profibus and CANopen, should, therefore, be available.

It is worth noting that these network connections can do more than simply provide user access to data. They can also allow the data to be accessed by the installation's control system – typically a PLC – which can be programmed to react automatically to impending and actual trip conditions. In addition, MCCBs can often be closed or tripped via the network and, in some cases, their trip settings can be adjusted in this way.

Network connections can also be arranged to allow MCCBs to be accessed remotely via, for example, a modem link or even an Internet connection. In this way, their operation can be monitored from almost anywhere in the world. With maintenance so often outsourced these days, this is a particularly important feature as it allows the maintenance company to make initial fault diagnoses without even having to visit the site.

All of the options discussed in this article were taken into account in the development of Moeller Electric's latest NZM range of MCCBs. Available with ratings from 20A to 1600A, and with breaking capacities from 25kA to 150kA, these competitively priced breakers offer a full range of communication and data management options.

They fully support the emerging FDT (field device tool) standard for remote diagnostics, and can be accessed from PCs running Moeller's XPC-SOFT software. The NZM range also includes an easy-to-use data terminal, which is suitable for hand-held use or panel mounting.

The main function of MCCBs is and always will be to provide reliable and effective circuit protection. As we have seen, however, recent developments mean that specifiers and users alike are now entitled to expect much more than this from their breakers!

This page last updated: 17 February 2006