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MCB INTERNAL VIEW


Box terminal 37 Hammer action solenoid Arc chamber


Trip bar Arc runner Thermo-element


Fixed contact Moving contact


Box terminal Busbar aperture DIN clip


GENERAL CONSTRUCTION


Wylex MCBs are of the thermal-magnetic current limiting type. MCBs have an easy to operate handle with a trip-free toggle mechanism – so even when the handle is held in the ‘on’ position the MCB is free to trip.


AMBIENT TEMPERATURE CONSIDERATIONS


Wylex MCBs are calibrated to meet the requirements of BS EN 60898, 30°C Ref Calibration Temperature. At other temperatures the following rating factors should be used:


At 60°C 0.9 At 20°C 1.0 At 0°C 1.1


Adjacent thermal-magnetic MCBs should not be continuously loaded at or approaching their nominal rated currents when mounted in enclosures. It is good engineering practice to apply generous derating factors or make provision for adequate free air between devices. In these situations, and in common with other manufacturers, we recommend a 66% diversity factor is applied to the MCB nominal rated current where it is intended to load the MCBs continuously (in excess of 1 hour).


METHOD OF OPERATION


1 Moderate overload conditions Detection of moderate overload conditions is achieved by the use of a thermo-metal element which deflects in response to the current passing through it. The thermo-metal element moves against the trip bar releasing the trip mechanism.


2 Short circuit conditions When the current flowing through the MCB reaches a predetermined level, the solenoid directly pulls in the plunger which forcibly separates the contacts and simultaneously releases the trip mechanism. 3 Establishment of arc between fixed and moving contacts As the moving contact moves away from the fixed contact, an arc is established. The arc runs along the arc runner to the arc chamber where it is split up between the plates and extinguished. The low inertia and consequent high speed of the moving contact has a limiting effect on the flow of fault current. The rapid development of the arc, together with its accelerated extinction in the arc chamber, BS EN 60898-2 gives a typical operating time of 3.5-5 milliseconds for a type B curve MCB.


CURRENT LIMITING ACTION


The high speed current limiting action ensures that the MCB operates before the full prospective fault current is allowed to develop. Under fault conditions, damage can be sustained to the installation and associated equipment due to the amount of energy that passes before the current is completely interrupted. The total energy let-through depends on the value of current and the time for which it flows, and is denoted by the symbol I2t. The high speed current limiting action of MCBs ensures that the energy let-through and any subsequent damage is minimised. This reduced energy let-through assists greatly with both back-up and discrimination considerations.


CURRENT LIMITING EFFECT


Maximum prospective fault


current


Healthy circuit current


Fault


20ms


20ms


MCBs - Technical Data


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