Handbook of Electrical Installation Practice - BeKnowledge

Motor Control Gear 465
Fig. 17.7 Typical small thermal overload relay covering FLC range of 0.14–40A direct
connected.
Electronic relays
Electronic overload relays offer several advantages over conventional thermal or
magnetic overload relays, including:
• Faster responses to three-phase overloads
• Faster response to phase loss
• Trip time–current settings are user adjustable
• Other features such as visual indication of overload current are available on
mid-priced units.
• Some models incorporate RS 485 communication ports.
A further advantage of electronic overload relays is that they dissipate less power
than an equivalent thermal or magnetic relay so there will be a lower temperature
rise in a starter enclosure where an electronic relay is fitted.
Magnetic relays
A magnetic relay employs a coil of wire in each pole of the motor supply; when
current is passed through these coils the resulting magnetic field attracts plungers
which when lifted actuate a trip switch. In order to cater for the starting current of
a motor a restraint device such as an oil dashpot is provided. When installing a magnetic
overload relay, care should be taken that the dashpots are filled to the recommended
level with the fluid provided by the manufacturer. If nuisance tripping