Front Cover: Matthew Short’s Shortwave heads south during the 2009 Rolex Sydney to Hobart Yacht Race. Photo by Howard Wright/IMAGE Professional Photography.
The purpose of circuit breakers is the same as that of fuses, i.e. to protect circuits when overload or short circuit conditions occur. Unlike fuses, they are not destroyed when an exception occurs and can be reset.
The two types of circuit breakers most commonly used on boats are thermal and magnetic-hydraulic breakers.
A thermal circuit breaker includes a bimetallic strip, which is a strip of material that has two different metals fixed together. The two metals expand with heat, but each does so at a different rate. The heat generated by the electric current flowing through the circuit causes the bimetallic strip to bend.
At excessive levels, the strip is bent to an angle that pulls the breaker’s lever down and severs the electrical connection. They are cheap to produce. Their greatest disadvantage is that they are affected by ambient temperature – they will trip with a lower current flowing through them when it is hot than when it is cold.
A magnetic-hydraulic circuit breaker has an electromagnet whose magnetism increases with the current load. The moment the load exceeds that which has been prescribed, the electromagnet’s pull becomes powerful enough to force the circuit breaker’s lever.
The hydraulic element in these breakers allows the designer to vary how soon after an overload occurs the connection is broken. These breakers are more expensive to manufacture than thermal ones, but they are less affected by ambient temperature, shock and vibrations.
One important feature of some hydraulic magnetic breakers is ‘trip free’ action, which means the circuit will trip in the event of an overload even if the levers are held in the ‘on’ position.
Designers can vary the time delay between an overload occurring and the breaker tripping. This enables users to select breakers that trip quickly as soon as excessive current flow occurs e.g. for electronic equipment. However, when protecting a circuit that includes an electric motor, one would want a breaker to only trip if an overcurrent continues beyond the spike that normally occurs during motor starting. A breaker with a longer trip delay would therefore be selected.
Having said that, if a catastrophically high current or a short circuit were to occur, all breakers will trip almost immediately. Manufacturers publish characteristic curves that define what delay occurs at what percentage overload. These are often given names such as ‘standard curve’ or ‘motor start curve’.
Some circuit breakers can only be turned on. There is no way of switching them off other than creating an over current or short circuit situation. There are called ‘reset breakers’.
Others have a lever that enables them to be switched on and off like a switch, or they have a reset lever plus a manual trip button that allows the circuit to be broken (‘switchable’ and ‘manual trip’ breakers).
Most breakers are designed to be mounted through a panel, with the actuator protruding to the front of the panel and the body and terminals behind the panel. Others are designed for surface mounting on e.g. a bulkhead.
Breakers are available with different types and sizes of terminals. It is best to use breakers with screw connections and closed ring cable terminals. The physical size of terminal lugs and the weight of the cable must be taken into account when selecting breakers for high current applications.
Breakers are available in ratings up to 150A. For higher current ratings, fuses should be used.
Breakers are rated to protect either AC or DC circuits or both. Although most AC breakers do also work with DC, stick to the manufacturer’s specification.
For AC installations, double pole main circuit breakers that switch both the active and neutral must be used. Regulations require that a breaker that trips if the polarity is reversed (relay trip breaker) must be fitted to boats. This gives protection in cases where incorrectly wired shore power leads are used. A residual current device (RCD) is also compulsory. An RCD detects any leakage to earth and instantly interrupts the power supply.
Once again, choose only high-quality components for circuit protection. Your valuable investment and life could depend on it. You can rely on breakers from manufacturers such as Airpax, Bussman, Carlingswitch and ETA.
*Gavin Sorrell works in collaboration with Aquavolt Electric Boat Parts.
Tel: 02 9417 8455 www.aquavolt.com.au
from readers. Manuscripts should be typed, double-spaced and on one side of the paper only. Black and white or colour photos preferred. All material should be accompanied by a stamped self-addressed envelope, if you want it returned.
Original content on this website are copyright and the exclusive property of AFLOAT Publications Pty Ltd. No part of this publication may be reproduced or transmitted in any form or by any means without the written permission of the publisher. The comments and views expressed on this site are not necessarily those of the publisher or editor.
