'Finely tuned' circuit protection
What is a 'circuit breaker for equipment' and how does it differ from
other types of circuit protection? Jonathan Adams provides some answers
A 'circuit breaker for equipment' (CBE) is a special type of circuit
breaker that provides carefully matched protection for an individual
piece of equipment, including electronic, electrical, communications and
industrial systems. Their use enables reductions to be made in wiring,
connectors and other components, resulting in lower costs and lower
weight. In terms of protected equipment value, moreover, they can help to
reduce downtime and warranty costs.
HINTS AND TIPS FOR THE DESIGNER
? The time-current characteristic plots tripping time against
over-current, expressed as a percentage of normal rating.
? Temperature compensated CBEs are available for applications where
stable characteristics are required over a varying temperature range.
? Very low voltage circuits may require the use of special models to
reduce power loss difficulties.
? CBEs for aerospace, defence, marine, industrial and automotive products
need to operate successfully under conditions of shock, vibration,
humidity, water splash and temperature extremes.
? Using CBEs with LVD certification enables compliance with European CE
requirements.
? Remember, many applications now require under-voltage as well as
over-current protection to ensure that equipment is not automatically
re-powered after a power failure or other disconnection.
So, what differentiates a CBE from other circuit protection devices?
Fuses and miniature circuit breakers mainly guard against catastrophic
failure - high short-circuit currents that might otherwise cause fires or
serious installation damage - and are generally not designed to protect
against low-level over-currents, the effects of which may be equally
serious. CBEs, on the other hand, provide precisely tuned protection,
allowing tripping characteristics to be specified to deliver the required
protection. Their high sensitivity protects against harmful overloads
such as slowly increasing currents caused by gradual circuit or component
failure or equipment malfunction or abuse, while allowing normal current
surges such as switch-on transients.
There are several different types of CBE, which can be specified
according to the type of equipment being protected. Thermal types, for
example, provide a positive snap action, which is suitable for motor and
transformer windings, automotive and marine installations, and battery
powered equipment. Hot wire thermal CBEs can be used with printed circuit
boards or any other application requiring fast switching, while magnetic
versions are available for sensitive electronic systems that need
extremely fast and precise tripping characteristics.
Thermal magnetic CBEs combine delayed thermal operation for low-level
over-currents, with fast magnetic operation on higher level overloads.
Temporary current surges can be absorbed without over-specifying the CBE
or degrading its sensitivity to even small sustained overloads.
Communication and control systems are typical of the high value capital
equipment that can benefit. If a more precisely defined delay is required
before tripping, then a hydraulic magnetic CBE is the best choice. Here,
the damping effect of a solenoid provides the delay and eliminates the
effect of short-term surges.
Jonathan Adams is managing director of E-T-A Circuit Breakers
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