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POWER GENERATION, TRANSMISSION & DISTRIBUTION 17

wherever a Type AC or Type A device is

specified, as they provide the same functionality

as these types and more.

But what of Type S RCCBs? These are selective

devices intended for use where circuits include

more than one RCCB. For example, an

installation could include an upstream RCCB to

provide protection for several downstream

circuits, some of which also have their own RCCB

or RCBO. If an earth fault occurs on one of the

downstream circuits with an RCCB or RCBO, this

device should trip rather than the upstream

RCCB. Using a Type S RCCB for the upstream

device will provide the necessary selectivity.

Now lets turn to RCBOs. In principle, as these

are functionally an MCB and an RCCB in the

same device, it would be possible to produce

versions with a huge range of combinations of

MCB and RCCB characteristics. In practice,

manufacturers limit themselves to the mostcommonly used combinations, such as a Type B

or C MCB characteristic with a Type A or AC

RCCB characteristic. Where other combinations

are needed, it often means that an RCBO cant

be used and that the necessary protection must

be provided by using a separate MCB and RCCB.

Some manufacturers, however, offer field-

fittable residual current units that can be used to

convert an MCB into an RCBO, and this makes

possible a wider range of combinations.

Two interesting new product developments

are worth mentioning at this point. The first isthat in addition to standard RCBOs, which are

bigger than an ordinary MCB, leading

manufacturers are now offering compact

RCBOs that save space in consumer units or

distribution boards and are easier to wire. These

compact devices are usually available with the

same combinations of characteristics as

standard RCBOs.

The second development is the recent

introduction of RCCBs utilising digital electronic

technology, which offer a very wide range of

characteristics and are particularly suitable for

use in difficult applications, such as the

protection of circuits supplying large numbers of

fluorescent lamps with electronic ballasts.

This article has looked only at operating

characteristics for protection devices and is

intended to provide guidance on choosing the

best type of device for use in specific

applications. When choosing individual devices,

however, there are many other factors that must

be taken into account including the current

rating for MCBs and RCBOs, the sensitivity for

RCCBs and RCBOs, and the short circuit capacity

for all devices.

Theres no room to cover these factors here,

but comprehensive guidance can be found in the

IET Wiring Regulations to which reference should,

of course, be made when designing any type of

electrical installation. Additionally, leading suppliers

of protection devices, such as Eaton, are always

happy to provide expert advice and guidance.

XEffect circuit breakers

MCB Operating Characteristics for Type B, C and D

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DRIVES AND CONTROLS

The MoD has also shown an

inclination to move away from

hydraulic to electric actuation, and

manufacturing companies like BAE now see the

future in all-electric actuation because of

reduced fire risk oil, leaks and reduced

maintenance. In the US, the Office of Naval

Research has already stated, The move to

integrated all-electric designs will significantly

improve efficiency, effectiveness and

survivability while simultaneously increasing

design flexibility, reducing costs and enhancing

quality of service.

The key benefits of servo electric actuation

are reduced cost, elimination of the need for

sophisticated hydraulic controls and simpler

logistical setup and maintenance.

Furthermore, there is no hydraulic fluid

pumping system, which is susceptible to leaks

of flammable and carcinogenic oil, and has

been the cause of numerous aircraft incidents

over the last 10-20 years.

Electric actuation makes military vehicles,

crafts and vessels less vulnerable in combat. In

aircraft, for example, it does away withhydraulic lines in fuselage and wing box, is light

weight and facilitates smaller aircraft and

unmanned aircraft design.

ALL-ELECTRIC DESIGN

Increased efficiency is another major benefit as

there is only one energy conversion process. The

trend towards all-electric design is fuelling this

development. On ships for example, the prime

mover, such as a fuel-burning turbine, will

convert the energy directly to electricity for

powering motors and actuators around thevessel, instead of conventional mechanical

power transmission, including the motors

driving the propellers.

Soon all the hatches, valves and firing

mechanisms will be electrically powered as well.

According to US publication Military and

Aerospace Electronics, electric power is already

the solution for industrial applications that need

tremendous torque. Designers of military

vehicles, in fact, see electric power as the nextgreat frontier.

The IEEE (The Institute of Electrical and

Electronics Engineers) has also stated there is a

general move in the aerospace industry to

increase the amount of electrically powered

equipment on future aircraft. High power

electric actuation systems are being proposed on

many new aircraft. Most new aircraft designs

now use electrically actuated ball screws drivenby electric motors for the spoilers, some flaps

and flight controls instead of hydraulic motors.

Installation time and cost is one of the

biggest issues of hydraulics, because piping a

kilometre or two of hydraulic hose requires

minimum radius bending and screwing in for

attachment to bulkheads in order to withstand

vibration. In contrast, an electric cable simply

attaches by means of cable ties and is easily fed

through from one space to another.

The latest US surface fleet features a number

of aircraft carriers sporting several hundred Exlarelectric actuators for hatches, loading

mechanisms, jet blast doors, deflectors, lifts,

firing pins and winches. In the UK, Exlar

actuators have found applications in a Queen

Electrical Review April 2013

18

At a recent conference hosted by Rolls Royce in Filton, the company revealed a roadmapthat clearly defined its vision for both its marine and aircraft divisions for the next fiveyears. This roadmap demonstrates a move away from hydraulic actuation and towardselectromechanical actuation. Here Piers Olsen, of Olsen Engineering UK, explains whythis is a sensible choice for design engineers across the board

Electric actuationmakes military

vehicles andcrafts less

vulnerable incombat

HYDRAULIC ACTUATION

MAKES WAY FOR ELECTRIC