Truths about UPS safety

February 13th, 2014, Published in Articles: Vector


by Bob Lauricella and Zach Greenhaw, Eaton

This article answers some common questions about UPS maintenance and how to reduce the risks associated with servicing UPSs and their batteries.

eaton-inst-tech-fig1When working with data center and commercial facility power systems, shocks of 100 to more than 2000 mA are possible. Energised electrical equipment also presents the risk of arc flash – the energy release that occurs during an electrical fault when current flows through the air between two live conductors, causing a short circuit. In a residential setting, arc flashes usually produce little more than a brief flash of light before extinguishing themselves harmlessly. In a commercial or industrial setting, however, voltages and current are much higher, so arc flashes routinely produce powerful explosions marked by searing heat, toxic fumes, blinding light, deafening noise, flying shrapnel and massive pressure waves.

Commercial and industrial electrical systems such as UPSs and their batteries require preventive and restorative maintenance, which usually requires technicians to be working near the unit, even inside it, and sometimes when it is still energised.

Fortunately, there are ways to minimise the risk to your employees, the equipment and the field technician performing the service.

What types of service do UPSs need?

Unplanned outages can quickly add up huge amounts in lost revenue – or even lost lives. Preventive maintenance improves the performance, availability and service life of this critical equipment.

UPSs which receive no preventive maintenance have a 400% greater chance of load loss compared to properly maintained UPSs. Analysis of millions of operating hours for thousands of UPSs has shown that mean time between failures (MTBF) for UPSs which receive preventive maintenance twice a year is more than 20 times better than for UPSs which receive no preventive maintenance. Increase the frequency of preventive maintenance to quarterly, even monthly, and UPS reliability and service life just keep getting better.


Proper maintenance will prolong the life of a battery and will help enable the it to satisfy its design requirements. A good battery maintenance programme will serve as a valuable aid in maximising battery life, preventing avoidable failures, and reducing premature replacement.

Repair service addresses the reality that even the best-made components eventually wear out and must be replaced. Capacitors, circuit boards, fans, batteries and power supplies are all consumable parts which you can expect to replace in a UPS periodically. A well-designed UPS configuration simplifies this process to minimise mean time to repair (MTTR) and restore function as quickly as possible.

Actions during preventive maintenance

The field technician will perform a variety of checks and inspections, both visual and with diagnostic tools. For example, infrared thermography identifies loose connections which show up hotter than good connections. Visual inspection of connection points shows evidence of soot marks or degradation which could signal a short circuit or bad connection. Computer analysis of the error log and alarm history can point to impending problems such as a loss of insulating value on a cable.

Following industry guidelines, a customer engineer may conduct the following preventive maintenance activities:

For the UPS

  • Measure and record input AC voltages and currents on all three phases.
  • Measure and record delta voltage at the static switch.
  • Measure and record rectifier output DC voltage and adjust if needed.
  • Measure and record AC voltages and currents on all three phases.
  • Test static switch for “no break” transfer.
  • Test all alarms for proper operation.
  • Measure and record all DC power supply voltages and adjust if needed.
  • Test all indicator lamps and report any which are not operating.
  • Inspect all ground connections and wire insulation for electrical safety.
  • Vacuum clean its interior.
  • Inspect air filters and replace if needed.
  • Inspect input and output conductors and tighten to the manufacturer’s specifications.
  • Document each inspection done along with actions taken or recommendations made.

For UPS batteries

  • Check the appearance, safety, cleanliness and temperature of the battery room.
  • Ensure that ventilation equipment is deployed per the manufacturer specifications.
  • Test each battery with a 100 A load applied for 10 seconds.
  • Measure and record the cell voltage and specific gravity of each battery cell.
  • Re-torque cell connections and micro-ohm to manufacturer’s specifications.
  • Measure internal impedance of all cells to the manufacturer’s specifications.
  • Check for corrosion on battery terminals and advise whether teardown is needed.
  • Check batteries for cell leaks or cracks and repair or replace as necessary.
  • Check battery cables and clean, repair or replace as needed.
  • Check the integrity of rack mounting, tighten as required, and notify of any structural damage.
  • Inspect the charger and inverter.
  • Check grounds, connections, wire insulation and electrical safety, and repair as needed.
  • Calibrate ammeters and voltmeters to the manufacturer’s specifications.
  • Calibrate charger DC output current to the manufacturer’s specifications, float and equalise voltage.
  • Calibrate inverter AC output current to the manufacturer’s specifications, float and equalise voltage.
  • Calibrate timers, alarm relays and indicator lamps to the manufacturer’s specifications.

IEEE 450-2010 outlines even more comprehensive guidelines for monthly, quarterly and annual maintenance.

Who performs this service?

The “dead front” in a UPS is the unfortunate nickname for the galvanised steel plate behind the attractive black faceplate of a data center UPS.

When you remove the front panel, this “dead front” is what separates people from the electricity. If that plate is removed, the UPS must be shut down fully, with no chance of it being energised by a secondary source.

The more complicated the equipment, the more important it is to have experts perform the maintenance to keep it running smoothly. Without the right safety training, and expensive (and often complex and proprietary) diagnostic, analysis and connectivity tools, UPS owners can perform only very limited service themselves. For example, a technician might be able to scroll through the alarm history and see voltage fluctuations, but not gain clarity into why these events are happening or know what to do about them.

Because large UPSs carry dangerous electrical voltages, all types of service to these units present the potential for personal harm and property damage to anyone without extensive safety training.

Even powering down the unit is complex. Consider that UPSs and their surrounding electrical infrastructures are designed to prevent the power from going off, so even if the main power source is switched off and the UPS itself is turned off, the equipment could still be getting power from other sources such as battery, generator or secondary utility feed. Consider also that some procedures such as infrared thermography and measurements of voltage, currency, resistance, and capacitance must be done with the unit fully operational.

UPSs under warranty or extended service contracts after the warranty period receive recommended preventive maintenance and repair services from authorised, trained field technicians. For smaller single-phase UPSs, manufacturers typically have authorised and trained independent contractors to provide this service. For the larger three-phase UPSs you can expect the field technician to be factory-trained and certified by the manufacturer.

In rare instances, UPSs are in remote locations where the only practical solution is for the manufacturer to train the customer’s employees to service the UPS themselves. In these cases, it must be configured so that everything in the UPS cabinet can be completely de-energised.

The field technician

Wear protective gear: even when working with a de-energised unit, the field technician should be wearing non-conductive safety glasses, safety clothing designed to withstand heat up to 8 cal/cm², and leather safety boots. Working with an energised unit requires either Level 2 (bodywear rated to 8 cal/cm²) or Level 4 (protection rated to 40 cal/cm²), depending on calculations. At Level 4, the field technician will also wear a 40 cal/cm² arc flash protection hood, hearing protection and two sets of gloves. Anything that exceeds 40 cal/cm² requires an escalation in safety level because the energy associated with a possible blast is extremely dangerous.

Personal protective equipment requires increased supervision and its effectiveness may be limited. For instance, a hardhat with tinted face shield protects the face but does not offer side impact protection. Nitrile gloves provide chemical resistance but do not protect against punctures and abrasion. Defects or excessive wear can inhibit the effectiveness of any gear. Gloves should be inspected daily. Hardhats should be replaced every five years.

How to minimise the risk

Prepare ahead and keep good records. Keep single-line diagrams up-to-date, so there’s a reliable record of the facility’s electrical system. Hold pre-job meetings and brief employees on their roles and safe work practices.

Implement an employee training programme to ensure data center personnel understand arc flash dangers and how to avoid them. Be sure every existing and new employee receives this training.

Configure each UPS with a maintenance bypass. This provides an alternate power path which bypasses the UPS circuitry altogether. The field technician can then service or replace the UPS safely, without interrupting power to loads.

Smaller UPSs, in the range of 2 to 30 kVA, may have an internal maintenance bypass located on or inside the UPS cabinet. When the technician switches to maintenance bypass operation, almost all of the UPS cabinet is de-energised. The portion of the UPS cabinet which is still powered – where the maintenance bypass resides – is shielded behind metal baffling or a metal wall. The engineer has safe access to the most frequently serviced components and can do fairly extensive work without undue risk.

For larger UPSs, loads are generally so critical that an external maintenance bypass is used, usually in a wall-mounted cabinet. Because the bypass path is physically separate, the entire UPS cabinet can be completely de-energised and field technicians can work with confidence.

A maintenance bypass is not required but it is strongly recommended on every hardwired UPS, even when the UPS has been configured for redundancy. It is a relatively simple device which provides a strong benefit for almost any application from 2 kVA up.

It is the safest way for field technicians to work on the unit, the safest configuration for your employees, and the safest way to protect the equipment and the load.

Implement arc flash reduction strategies. Every arc flash mitigation programme should begin with a hazard analysis – usually performed with help from a qualified power systems engineer – to calculate the amount of energy an arc flash could release at various points along the power chain.

Facilities managers can dramatically mitigate arc flash likelihood and severity by:

  • Reducing the amount of available fault current.
  • Decreasing the time required to clear faults.
  • Anticipating and eliminating the conditions which cause faults.
  • Redirecting the blast force of an arc flash through the use of arc-resistant switchgear.

The arc flash hazard risk category cannot be determined accurately until the energy potential is evaluated in context. The parameters of a potential arc flash event – how much force/fire is likely and how long it will last – are functions of how much power is being connected in the unit and how quickly the interruption device (such as fuse, breaker or the UPS itself) will activate. Since this risk rating can vary for each aisle of a power setup, manufacturers should not apply arc flash labels to equipment as it ships out. Onsite evaluation is available as a service once the equipment is installed.

Companies should mark arc flash hazard zones on the floor so workers not wearing PPE can see clearly how far they must keep away from electrical equipment to avoid potential injury.

Safety practices of service vendors

UPS service vendors must be able to show you several tangible measures of their safety performance and commitment as evidenced in safety practices and reportable incidents.

Recordable incident rate

Incidence rates can show the relative level of injuries and illnesses among different industries, firms or operations within a single firm. These rates can help determine both problem areas and progress in preventing work-related safety incidents because a common base and a specific period of time are involved.

Training and safety protocols

Eliminating safety incidents is a matter of recognising and evaluating the risks, then implementing the correct controls to prevent or mitigate them. Review the service provider’s procedures related to insurance, employment checks and safety programmes in the previous three years, management systems and initial and refresher training for field technicians.


The risks associated with high-energy voltages are serious but there are many ways to prevent incidents or reduce their effects. Prevention is foremost; mitigation is the second line of defense.

Contact Marlene Coetzee, Eaton, Tel 011 824-7400,

Related Articles

  • Now Media acquires EngineerIT and Energize from EE Publishers
  • Fan range redesigned
  • Pneumatic, hydraulic, vacuum process and automation offering expanded
  • Machine Tools Africa 2020 to showcase market trends
  • New app supports predictive services for drive systems