Protection options proliferate: but traditional solutions still dominate the market

By Heiko Luckhaupt, Category Marketing Manager, Automation & Control

As electronic control techniques have spread throughout the automation arena, so the need to incorporate protection against electrical faults has increased. The components and subsystems used to do this are today often a far cry from the simple fuses and circuit breakers of old. As automation systems have penetrated more and more industries, system requirements have become increasingly sophisticated, leading to an expansion in the variety of protection functionality.

In response, manufacturers have come up with a wide variety of protection products that are easy to use and install, both for panel builders and for the people who operate and maintain the equipment. Often, products are modular or configurable, reducing the need to hold inventory of a wide range of very similar components. Like all electronic devices, modern solutions are becoming smaller in size, with the benefit in automation systems of reduced panel footprint.

In some cases, single-use fuses have been replaced by resettable solutions: productivity has also been improved by designing components that produce fewer false trips, and which contribute to fault-finding by providing an indication of where and how things have gone wrong. The concept of intrinsic safety has also had an impact, leading to the development of a range of techniques to avert problems in inherently hazardous areas.

In specialised areas such as motor control, manufacturers have also added the capability to easily combine overload protection with existing contactor and control products.


ABB’s S200 miniature circuit breaker

Despite this fast pace of change, the fundamental needs have remained largely the same over many years: to prevent injury to people, to protect other parts of the system from physical or electrical damage, and to eliminate fire risks. It should therefore come as no surprise that a major part of the circuit protection market continues to consist of traditional fuses and circuit breakers, for use in the power control and distribution subsystem.

In short, it is very hard to find a system that doesn’t incorporate a fuse of some sort.

The commodity nature of such components means that this is an area where supply chain issues such as price, availability and lead-time are of genuine importance.

By choosing a leading distributor such as RS, engineers get one-stop-shop access to a comprehensive circuit protection offer, which includes MCBs, RCDs, RCBOs, and earth fault detection products. The company offers more than 1100 circuit breakers alone, in addition to BS88 / IEC 60269 fuses and holders, as well as related components such as isolators, safety protected contactors and control products. To cover every application, the company stocks several leading brands of control gear.

Standards are essential knowledge

In a market such as this, one vital element for the engineer is familiarity with the relevant industry standards. As an example, the UK’s BS88 (IEC 60269 is the set of international standards for low-voltage fuses), covering cartridge fuses for use at ‘low voltages’ (up to 1kV AC and 1500V DC), must surely rank among the longest standing electrical standards, with roots that go back almost 100 years.

The other main family of global standards governing the area is IEC 60947, which covers a variety of equipment types for use at the same low voltage levels in the industrial environment:

  • IEC 60947-2 relates to circuit breakers, including moulded case circuit breakers (MCCBs), residual current devices (RCDs) and air circuit breakers (ACBs) and is the most generally relevant to the field of circuit protection. It includes special requirements for integrally fused circuit breakers, and devices that provide earth leakage or electronic over-current protection.
  • IEC 60947-3 covers switches, disconnectors, switch disconnectors and – again most relevantly for circuit protection considerations – fused combination units.
  • IEC 60947-4 applies to contactors and motor starters. It comes in three parts, covering electromechanical components, semiconductor solutions, and devices intended for non-motor loads, and contains the requirements for circuit breakers used as direct-on-line starters.
  • IEC 60947-5, -6 and -7 are less relevant in this context, being more concerned with issues such as machine safety, and products such as proximity switches and terminals.

MCBs continue to dominate

As we have already observed, fuses and circuit breakers continue to dominate the circuit protection market in terms of units sold. Typical solutions in the automation arena include ABB’s S200 and S500/800 series.

The S2xx is a well-established series of current limiting, DIN rail mounted miniature circuit breakers that are available in a range of different short circuit capacities. Like many MCB products, however, the numerical rating does not tell the whole story. The S2xx series is available with five different tripping characteristics, for use dependent upon the application.


The commonest ‘standard’ characteristic is the ‘C’ curve, which is used for general-purpose light and power applications. The ‘D’ and ‘K’ curves are most often chosen for motor starting and transformer protection purposes, where high inrush currents and inductive loads may be encountered.

Standard trip curves for the S200 series -- C, D and K


The ‘B’ curve has a quick trip characteristic designed to protect cables and control circuits under low short circuit fault levels. Finally, with instantaneous tripping between two and three times rated current, the Z curve can be used to protect very sensitive devices.


Standard trip curves for the S200 series -- B, Z


Standard trip curves for the S200 series -- UC-K, UC-Z


The more advanced S800 series devices include a number of high-performance features that make them particularly suitable for use in group protection. For instance, the S800-SCL-SR is a self-resetting current limiting module designed to restrict the short circuit current flow until the downstream means of protection trips.

The Siemens VL Series is a good example of the move towards a flexible, modular approach to MCB specification, intended to reduce inventory requirements and cut downtime. Seven frame sizes are available, which can be populated with a choice of three types of interchangeable trip units: thermal-magnetic, electronic or electronic with LCD.

To accommodate the wide variety of requirements in terms of connecting, mounting and operating the components, Siemens offers a range of external accessories. The modular nature of the system also allows installations to be upgraded or re-rated on-site.


Siemens’ VL series MCBs


Like ABB, Schneider / Merlin Gerin provides a wide range of MCBs with a variety of trip curves available. Part of the company’s Multi 9 system, the company’s C60 and C120 circuit breakers can be supplemented with extra functions such as shunt or under/over-voltage release. Users can also choose to add a Vigi module, which incorporates a residual current relay and toroid in a case, to provide protection against insulation faults. The module is available in instantaneous or selective trip versions and is protected against nuisance tripping due to transient overvoltages caused by events such as lightning strikes or switchgear operating nearby.


The Merlin Gerin Vigi module

Motor control is a special case

Faults in motor control applications can be the result of both electrical and mechanical problems. These range from short circuit conditions, a locked rotor or shaft bearing seizure to a slight overload, abnormal temperature rise or ventilation fault. In these situations, special protection is therefore usually built into the motor control system itself, in the form of an overload relay.

A good example is the Schneider TeSys D-line series. Contactors are available in both AC and DC versions in compact 45mm and 55mm sizes (40A to 65A). As well as motor starters, they can be used to power control and signalling equipment. An overload relay such as the Tesys LR97D can be directly mounted beneath the contactor, to provide a range of protection functions. These include preventing over-currents caused by overloads or mechanical jamming; starting and steady-state operation over-torque monitoring; and torque surge protection.

The LR97D also includes specific protection for locked rotor and phase failure conditions. The current threshold is user-adjustable with separate time settings for starting and steady-state operation, and the device provides diagnostics via two LED indicators.


The Schneider TeSys-D series motor starter devices

Dealing with hazardous environments

Finally, it is worth noting that dealing with fault conditions needs special attention in hazardous environments, where sparking or overheating may have disastrous consequences. Of particular concern is the fact that both signals and power often need to pass between hazardous and non-hazardous areas without creating a risk.


The MTL7700 Series Zener barrier

The answer in these cases is to include a safety barrier that can divert any excess energy to ground. The MTL700 and more-recently announced MTL7700 devices are typical examples, which use a combination of Zener diodes, resistors and fuses to protect hazardous areas.

Available in one- or two-channel variants, the devices are used in series with signal transmission lines, passing an electrical signal in either direction without shunting it, but limiting the transfer of energy to a level that cannot ignite explosive atmospheres. They enable a wide range of measurement and control operations to be carried out simply and inexpensively by intrinsically safe techniques.


Designers have more choices than ever for providing electrical protection in process control and automation environments. These techniques extend from the power control and distribution subsystems right into the data and signalling path.

No other area provides quite such a proliferation of products, ranging from the simple to the very sophisticated. It is easy to see why: although often neglected, these components are both a first line of defence, and a potential source of downtime. Although sometimes neglected, engineers would do well to understand the options facing them, and select their protection strategy carefully.