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Backfeed protection:
The safest way to avoid any nasty shocks
Jason Yates of Riello UPS highlights the importance of backfeed protection in UPS systems and warns against the growing trend of corner-cutting with this vital safety mechanism.
ny device depending on electricity brings with it a certain degree of danger in terms of potentially hazardous shocks and arc flashes. Uninterruptible power supplies are no different in that respect. That’s why they incorporate backfeed protection to safeguard the health and wellbeing of any service engineer or other personnel working on the unit. Typically, UPS manufacturers have included complete and comprehensive backfeed protection devices as standard. But there’s a recent tendency from some manufacturers to shift away from this norm, which puts electrical installers and contractors in the difficult position of being ultimately responsible for ensuring the appropriate protection is in place.
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Backfeed basics Backfeeding relates to electricity travelling in the reverse direction to its usual flow of power. Say there’s a mains supply failure or a fault with the UPS. In these circumstances, the current could start feeding back from the UPS to any isolated circuits. For example, if a bypass supply thyristor
12 Electrical Review | May / June 2021
short circuits, the output from the inverter could pass through to the input terminals. Now, without the necessary backfeed protection, this would be extremely dangerous to anyone handling that circuit. But with the appropriate preventive measures in place, a service engineer can safely work on the incoming supply side of the UPS without the risk of suffering an electric shock from any current backfeeding from the output. So what specific safety regulations apply to UPS systems and backfeeding? The quality standard BS EN 62040-1:2019 Uninterruptible Power Systems (UPS): General and safety requirements for UPS stipulates that backfeed protection devices, ‘must be capable of preventing hazardous voltage or energy from being present on the UPS input AC terminals.’ Looking into further detail, the standard states that no shock hazard should be present at the input terminals one second after de-energisation on smaller, plug and play UPS, or after 15 seconds for hardwired UPS systems. The EN standard also outlines two locations where you can install