5.5 Fuses and circuit breakers Fuses are generally located in the distribution board. Each AC circuit (group) is fused separately. The fuse is matched to the size of the expected load. The fuse protects against: • Overload - when more current runs in the system that can be normally expected. • Short circuit - when the phase conductor accidentally comes in contact with Neutral or Earth.
Traditionally a fuse contains a wire that melts when unacceptable current passes through. As soon as the wire in the fuse has melted the electrical circuit has been broken and no additional current will flow. But, more common are overcurrent protection devices called: “Miniature Circuit Breaker (MCB)”. This device has two triggers for activating its switch-off mechanism. Being a thermal trigger for long term small overload currents and a magnetic trigger for large short duration currents like short circuit currents.
MCBs come in three types: B, C and D. They all have the same thermal characteristics. But they have different short circuit current levels. • Type B disconnects at 5 In (5 rated currents) and is commonly used as a household MCB. • Type C disconnects at 10 In and is used for transformers and fluorescent lamps. • Type D disconnects at 20 In and is used for large motors, transformers and mercury lamps. When a short circuit current occurs, with sufficient current, the MCB (B,C or D) is switched off within 100 ms.
5.6 AC bypass switch
It is recommended to add a manual bypass switch to an inverter/charger system. This is especially useful in mission critical systems. This switch allows to bypass the inverter/charger and connects the AC input (grid or generator) directly to the loads.
A switch like this will prove invaluable in case the inverter/charger needs a configuration change or should anything go wrong with the inverter/charger and it needs to be removed for service.
The bypass switch will need to break the AC in and AC out path to and from the inverter/charger and it then needs to make the bypass circuit. The switch need to be rated to the full AC load of the system.
5.7 Special considerations for AC wiring of parallel and/or 3 phase inverter/chargers
AC fusing parallel strings For units in parallel use one AC fuse for all units on that phase. Both on the input, and on the output. Multiple fuses which are mechanically connected are okay to be used, this is counted as one fuse.
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