Electrical Review: Backfeed Protection In UPS Systems (White Paper)

We explain why backfeed protection is a vital safety mechanism and warn against the growing trend of corner-cutting.

Backfeeding refers to electricity flowing in the opposite direction to its usual flow of power. For example, if there’s a mains failure or fault within the UPS, the current could start flowing back from the UPS to any isolated circuits.

Without the appropriate preventive measures in place, any service engineer handling these circuits is in danger of exposure to dangerous electric shocks or arc flashes.

In the May-June edition of Electrical Review, we explore the concept of backfeed protection in UPS systems. This is our latest white paper.

We start off by examining the relevant regulations outlined in the quality standard BS EN 62040-1:2019 Uninterruptible Power Systems (UPS): General and Safety Requirements for UPS.

This stipulates that a UPS must incorporate backfeed protection devices that prevent hazardous voltage from the input AC terminals either one second after de-energisation (for pluggable UPS) or 15 seconds after de-energisation (for hardwired UPS).

Different Types Of Backfeed Protection

According to BS EN 62040-1:2019, backfeed protection is permitted in two locations: internally inside the UPS itself or an external isolation device on the input supply line. The latter is only applicable for hardwired systems and would entail a magnetic contactor or circuit breaker.

Our article takes a closer look at internal devices, starting with “plug and play” UPS, typically 1-phase with an input of up to 16A.

In these cases, the backfeed protection device must ensure complete disconnection of the live and neutral input conductors using an air gap. Usually, this comes in the form of a relay that opens if there’s a mains failure.

For larger hardwired UPS (input rating greater than 16A), internal backfeed protection is either mechanical or electronic.

The former is similar to the relay or contactor-based approach highlighted above with pluggable UPS.

Electronic backfeed detection involves continuously monitoring the flow of current through the bypass line. If it detects a fault in the bypass thyristors, it immediately powers down the inverter.

Whatever type of backfeed prevention, you must place warning signs stating “Risk of Voltage Backfeed” or similar on any isolators installed upstream of the UPS.

Don’t Cut Corners

Riello UPS includes complete internal backfeed protection across our entire range. But many other manufacturers don’t. In fact, there’s a trend not to include such protection as standard.

This places the ultimate responsibility to comply with BS EN 62040-1:2019 safety standards with the electrical contractor or installer. A danger with this is that they may not have the necessary product-specific expertise.

For that reason, we urge anyone looking to specify a new UPS to clarify whether they’ll also need to install any additional backfeed protection.

UPS with ready-installed and fully-tested backfeed prevention are the best option to ensure you meet all safety considerations.