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The importance of Surge Protection Devices (SPD) when protecting an uninterruptible power supply (UPS)

“I have a UPS, why do I need an SPD?” For over 15 years this has been a common objection dealt with on a regular basis. Following are some facts about UPS systems and their susceptibility issues, as well as some comments from IEEE.

A UPS provides secondary or backup power in the event of loss of utility power. A UPS protects electrical and electronic systems, process controllers, and data against the loss of the primary power source, which is the utility company.

Many UPS’ incorporate relatively low energy and low pulse life Category A or B surge suppression. This low energy built-in surge suppression may protect the UPS and its connected load from a limited number of small hits, but it should not be relied upon as a surge transient solution. The Institute of Electrical and Electronics Engineers (IEEE) recognized this problem that a networked Transient Voltage Surge Suppression device and UPS TVSS protection is required.

There are two basic types of UPS systems. First is the rotary UPS which has a motor connected to the primary power source, which is used to turn an alternator which generates the AC output to power selected loads, along with a battery bank. The other type is the electronic or static UPS. This type of unit relies upon inverters and rectifiers to generate the AC sine wave and is supported by a bank of batteries to achieve the desired run times. Within this category you find stand-by, line interactive and double conversion types of UPS’.

Transient Susceptibility of the UPS
  • Protecting the Rotary UPS: Surge protection is recommended at the input to the rotary UPS to protect the motor windings, (and rectifier/charger and inverter when present), static switch, and bypass path to the loads. Additionally, protection at the output is recommended to protect the sensitive loads from the inductive transients developed in the generator inductance and/or inductive loads served by the rotary UPS as these loads are cycled on and off.
  • Protecting the electronic UPS: Surge protection is recommended at the input to all electronic loads and the UPS is no exception. The AC-to-DC converter, the static bypass switch, and to a lesser extent the DC-to-AC inverter, are all susceptible to transient damage. Sensitive and transient susceptible electronic control circuitry constantly monitors the status of the UPS and the input and output AC power of the UPS. This circuitry is used to determine if the UPS should be delivering power from the batteries and DC-AC converter: or, if the UPS should switch to the bypass mode and deliver AC from the secondary source which is usually the utility AC power. Additionally, when the UPS is in the bypass mode for maintenance or due to failure, the sensitive loads should also be protected against transient disruption, degradation, and damage from the transients appearing on the secondary AC source or utility power.


The IEEE Standard Section 9.11 states that networked SPD protection is needed, and UPS describes SPD protection as follows:
  • Service entrance protection “… A listed and properly rated TVSS should also be applied to each individual or set of electrical conductors (e.g. power, voice, data) penetrating any of the six sides forming a structure.”
  • Premise electrical system lightning/surge protection “… it is recommended that additional SPD be applied to downstream electrical switch boards and panel boards if they support electronic load equipment.”
  • UPS surge protection UPS surge protection: “Lightning and other transient voltage producing phenomena are harmful to most UPS equipment and to sensitive electronic load equipment (e.g. via an unprotected static-switch bypass around the UPS). Therefore, it is recommended practice that both the rectifier-charger input circuit to the UPS and the associated UPS bypass circuits (including the manual maintenance bypass circuit) be equipped with effective Category B TVSS protection as specified in IEEE Standard Section 9.11
  • IEEE standard assumes that a TVSS protection network is in place when discussing UPS surge and transient protection.

The requirement that the service entrance and downstream switchboards and panel boards are properly protected with TVSS is assumed to be met. Additionally, Category B TVSS protection is required for the rectifier-charger input circuit and the associated UPS bypass circuits.

  • Built-in UPS transient voltage suppression: Today, many UPS manufacturers provide low level lightning and surge protection. A typical UPS designed to power a computer may claim the UPS passes ANSI/IEEE C62.41-2002 Category A and/or Category B test waveforms. In general, one would assume that the UPS survives these transients, since there is no pass or fail in the ANSI/IEEE C62.412002. The adequacy of this built-in surge suppression to protect the connected load(s), the let-through voltages, the energy handling capability, and the number of hits of a given amplitude for the various transient waveforms that the built-in surge suppression will survive are not in general provided by the UPS manufacturer.

If a protected load is of sufficient value to an organization to protect against power outages with a UPS, the UPS and the vital load should also be protected against transient disruption, degradation, and damage.


A Transient Voltage Surge Suppression unit placed online in front of the UPS will protect the investment in the UPS and the connected computer equipment. During maintenance and downtime on the UPS, the SPD will filter out the transients and surges from the line during the static bypass mode or maintenance by-pass mode of operation, thus protecting the connected computer equipment from damage. The UPS itself will benefit with fewer maintenance problems and less downtime.

With the investment in sophisticated computer equipment and a UPS unit, the customer has already committed to keeping the system operational on a continuing basis. The additional investment in the SPD is minimal by comparison. The additional protection can be significant in terms of preventing even one downtime incident, which at today’s costs can run some companies as much as $78,000 per hour according to studies conducted and published by Computer Technology Review magazine.

To learn more about Alltec’s surge protection devices (SPD), visit our website or

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