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Chapter 12 Helpful information
Earth leakage considerations using an RCD
This section examines one of the most common site issues with UPS systems, which is the
tripping of the earth leakage protection, Residual Current Device (RCD) also known as an Earth
Leakage Circuit Breaker (ELCB) or Differential (Lock) Circuit Breaker.
Overview
One of the most common site issues with UPS systems is the tripping of the earth leakage
circuit breakers ELCB / RCD. To examine this in greater detail, we need to look at this device:
ELCBs contain a core-balance transformer and work on the following simple principal:
If the current which goes out on the phase does not return on the neutral, or on another phase,
in the event of a three phase type, it will be detected as an earth fault through an imbalance in
the core balance transformer and this imbalance will activate a tripping coil so that the circuit
will be disconnected. Therefore, if one comes in contact with a live appliance, the current,
which flows through this person to return to earth, is sufficient to trip a 30 mA RCD instantly.
Principles of Operation of a Standard RCD / ELCB / Differential (Lock) CB
Schematic diagram of an RCD connected to earth fault showing the principle parts of
the installation.
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6U Rack or Tower UPS RT8kVA and RT11kVA IUOMG
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