When thye UPS Turns the Power Down

UPS systems are designed to protect the sensitive load when mains power is available and
continue to supply power in case of utility outage. The UPS senses its output voltage,
transferring to Battery mode in case of Backup or Line interactive UPS, or to Utility Power in
case of Online UPS.

There us however a possibility, where UPS failure causes total output outage. These type of
failure, although rare is generally attributed to a Common Point Failure. Common point failure
is defined as any single failure that may cause full power failure at the input of the protected
equipment, in spite of measures designed to provide uninterrupted power, either from the mains
by means of the transfer switch, or from another UPS in redundant configuration. In redundant
systems the term frequently considers only UPS systems ability (without By Pass) to protect the
load.

People are generally more tolerant regarding failures related with utility power outage. They don't
accept however situations that makes the case worse, when UPS failures interrupt power to the
critical load while mains exist.

Reviews indicate that human interference is the main reason for UPS Power failures. About 30%
of failures happen during UPS maintenance service. Occasional error by the maintenance
technician, or lack of acquaintance with the equipment and the necessity to isolate the maintained
UPS module, are the main causes of total power failures with stand alone or with redundant UPS
systems.

AC capacitor failures form a significant cause of single point failures in redundant
configurations. Each UPS has an AC capacitor at its output as a part of an LC output filter. A
short circuit of the AC capacitor, presents a short circuit on the common output of parallel
connected systems. Some sophisticated schemes, based on sensing power flow direction are
able to isolate the failed UPS by disconnecting its output switch. Unfortunately, this is not the
case with most systems.

Similar behavior can happen due to power IGBT failure in the Inverter. Shorted IGBT is
reflected almost as a short circuit, limited only by the impedance of the output chock, on the
common AC buss. In most cases this phenomena is easily detected by the failed UPS monitoring
logic, disconnecting it from the common buss.

Numerous cases report DC capacitors to be the source of total Power failure. Due to
overvoltage, aging or internal failure, DC capacitors may overheat, developing electrolyte boiling
associated with internal pressure. Large Electrolytic capacitors are equipped with pressure relieve
valves, but sometimes, as a user reports: " The internals of the capacitor were ejected from the
UPS at such a high rate of speed, that it dented the front door of the UPS itself and caused the
door to jump the lock and swing open.".

The associated damage sometimes causes shorts in other circuits, between circuits and from
input or output lines to ground.

Some of the said phenomena are associated with temporary load transfer to the mains, which
after a preset time is re-transferred back to UPS operation, when proper operation is resumed.
Unfortunately, a failed transfer switch is sometimes revealed only when it is called to action,
causing total output power failure. In some cases a faulty circuit breaker, fuse or loose
connection in the incoming power line, is revealed only when the UPS fails and transfers to By
Pass.

Last but not least, communication failures between UPS and operator's equipment or personnel
in certain cases avoid exposing UPS malfunctions, which if revealed in time, could be treated
before the power goes down.

The mentioned failure types are far from being exhaustive, numerous other failures, sometimes
dormant until revealed in specific circumstances, particularly associated with parallel operation of
UPS systems, may cause total AC bus to fail.

Total output failure, when utility exists, may be very costly not only to the user that suffers from
loss of power, but also to user - manufacturer relations, which may be totally disrupted, after
such total output fail incident.

The good news is, that firstly these types of failures are not common. In UPSonNet February
2009 Newsletter 34 power failures occurring due to stormy weather were analyzed, indicating
that no single case of UPS failure was recorded. Secondly, training, preventive maintenance and
proper information to maintaining crews can decrease dramatically and even avoid entirely total
UPS failures.

Manufacturers avoid generally from marketing viewpoint, discussing UPS failure possibilities.
This attitude impacts not only the unaware user, but also the maintenance personal, which lacks
information and skills that a good manual could have provided.

Most user manuals do not include preventive maintenance information, such as step by step
instruction aimed to isolate the defective module in order to eliminate the possibility of total
power failure due to human error, Instructions to monitor on a periodic basis the heat of AC and
DC capacitors, exercising transfers from UPS to the By Pass, and ascertaining that
communication from the UPS works and reaches the relevant personnel. These kind of
instructions, training of maintenance crew, and periodic maintenance, will enable in most cases
elimination of total power failures, at a negligent cost compared to the possible damage.

When the UPS Turns the Power Down