Hazard of backfeed on some UPS units | Resources Safety & Health Queensland
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Mines safety alert no. 264 | 18 April 2011 | Version 1

Hazard of backfeed on some UPS units

Recommendations

  1. That site procedures be reviewed to ensure UPS units and the systems they supply comply with the safety requirements of:
    • AS 62040.1.1-2003 Uninterruptible power systems (UPS)—General and safety requirements for UPS used in operator access areas, and
    • AS 62040.1.2-2003 Uninterruptible power systems (UPS)—General and safety requirements for UPS used in restricted access locations.
  2. That routine maintenance procedures involve checking the provision and operation of backfeed protection circuitry.
  3. That a 'test for dead' be conducted before handling areas that may be affected by UPS backfeed.
  4. That persons be immediately informed of, and all units on the mine site tested to control and manage, this hazard. Note that AS 62040.1.1-2003 (Annex L (normative) Backfeed protection test, section L.1) states that excessive currents aren't allowed between any pairs of a UPS input terminals during stored energy operation, although this measurement need not be taken if measured opencircuit voltage does not exceed 30 V r.m.s. (42,4 V peak, 60 V d.c.).

Investigations are ongoing and further information may be published as it becomes available. The information in this publication is what is known at the time of writing.

We issue Safety Notices to draw attention to the occurrence of a serious incident, raise awareness of risks, and prompt assessment of your existing controls.

Comments

Testing several units revealed varying backfeed voltages from identical units, as well as backfeed on a unit from another manufacturer. These units were not fitted with protective devices to limit or shutdown supply if a person received a backfeed shock. Another unit showed no backfeed, so did not present a hazard to anyone performing electrical work or simply unplugging smaller plug in units.

Mine type

All mine types

Incident

A recent electric shock at a mine has highlighted the hazard of backfeed on some uninterruptable power supply (UPS ) units. These apparatus provide emergency power to a load when the input mains power is interrupted or disconnected. However, in this case, 240Vac was measured on the plug pins of the disconnected UPS mains supply cable while the UPS was powering a load. Subsequent testing of other units on site revealed 75% of all models and brands tested had backfeed voltages of 55Vac or more.

Equipment

Various UPS systems.

Hazard

When the mains supply is isolated, any UPS backfeed voltage could possibly energise otherwise isolated circuits. As this backfeed voltage is unlikely to be covered by any protection circuitry, it could prove lethal to anyone coming into contact with this circuit.

Cause

Lack of backfeed protection as required under AS 62040.1.1-2003 Uninterruptible power systems (UPS)—General and safety requirements for UPS used in operator access areas, section 5.1.4 Backfeed protection.

Authorised by Gavin Taylor - Chief Inspector of Mines

Contact:

Issued by Queensland Department of Employment, Economic Development and Innovation

Placement: Place this announcement on noticeboards and ensure all relevant people in your organisation receive a copy.

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Acknowledgement of Country

RSHQ respectfully acknowledges all Aboriginal peoples and Torres Strait Islander peoples as the Traditional Owners and Custodians of the country on which we live and work. We acknowledge their continuing connection to land, waters and culture and commit to ongoing reconciliation. We pay our respect to their Elders past and present.