Proper coordination of protection devices is critical for effective operation.

Uninterruptible power supply (UPS) systems ensure critical loads have continuous and quality power. These systems can only accomplish this with proper application; involving design, installation, and maintenance. Making provisions for effective protective coordination schemes to protect the UPS and critical loads is an equally important aspect of all these systems.

Proper coordination of protective devices in the UPS distribution system means that only the fuse or breaker directly upstream of a fault (short circuit) will operate to clear the fault. For example, the fuse (Fuse 1) on the branch circuit of the UPS panelboard should clear the fault located on that branch circuit, as opposed to the panelboard's main breaker (Breaker 2) clearing this fault. As long as the fuse (Fuse 1) melts quickly (before the panelboard's main breaker senses the fault) the interruption of power is confined to the faulted load only. However, if the panelboard's main breaker were to operate, then all critical loads fed from the panelboard would lose power.

Protective device coordination involves evaluation of the system's available fault current and selection of current-interrupting devices that will operate in a coordinated manner with respect to time versus fault current.

The source of fault current to a fault downstream of a UPS system's static switch will initially come from the UPS. However, if the fault current available from the UPS is insufficient to cause operation of protective devices, the static switch will operate to transfer the faulted load to the bypass source. The bypass source will then supply the fault current required to cause the protective devices to clear the fault.

The inverter's overload capability limits the amount of current a UPS is able to deliver to a fault. The overload capability of UPS inverters varies, depending on the technology of the inverter.

The inverter will limit the current to protect itself from overload conditions, such as a fault at the UPS output. However, the static switch operates when the inverter gets near its current-carrying capacity. Unless the fault is cleared before the inverter current limit is reached, the static switch will transfer the faulted load to the bypass source.

The amount of fault current an alternate source delivers to the fault is typically higher than the amount a UPS is able to deliver. The higher fault current helps to ensure protective devices will sense the fault and operate to quickly clear it.

Assume a 20kVA UPS is a single-phase, 120V output, PWM unit. Based on an overload capability of 150% for PWM technology, the available fault current from the UPS system is 20kVA/120V x 150% = 250A. You should make sure to set the static transfer switch to operate before the UPS reaches its overload capability, such as at 225A.

Assume the maximum available fault current at the secondary of the isolation transformer is 2000A.

Fast-action 20A fuses were selected as the overcurrent protection of the branch circuits of the UPS panelboard. The UPS is capable of delivering 225A to the fault, but Fuse 1 will melt in 0.01 sec at 65A.

For a 1000A fault, Breaker 2 will begin to operate as quickly as 0.02 sec and will clear the fault in no more than 3.5 sec. There is a 480V, 60A breaker ahead of the isolation transformer (Breaker 1).

Take a look at the following example. For a 2000A fault at the secondary of the isolation transformer, Breaker 1 has a minimum sensing time of 2 sec and maximum clearing time of 6.5 sec. Breaker 1 would see 500A (2000A2120/480). The curves for the 20A breakers and the 100A main panelboard breaker overlap. This means that a fault located on a branch circuit may cause Breaker 2 to open.

You may encounter another problem when using breakers as the branch-circuit protection for UPS loads. By comparing the time-current characteristics for a 50A fuse and 50A breaker, you can see the UPS may be unable to deliver enough fault current to the 50A breaker to allow the breaker to operate.

As long as the alternate source is available, the static switch can transfer the load to the alternate source, which has greater fault current available. However, if the alternate source is unavailable, then you must rely on the UPS to supply the current required to allow the protective devices to clear the fault.

You will recall that the overload capability of the 20kVA UPS is 150% (or 250A) for 10 sec and 125% (or 208.3A) for 10 min. At 250A, it takes up to 10 sec for the 50A breaker to clear. A fault on a branch circuit that is sustained for 10 sec will interrupt power to all other UPS loads for 10 sec. However, the 50A fuse is able to clear the branch circuit fault in 0.01 sec.

The selection of fuses for UPS branch-circuit protection often provides better protective device coordination than breakers.


EC&M articles:

"Understanding UPS Techno-Babble," p. 80, Nov. '98;"UPS Stands on Firm Ground," p. 40, Nov. '99.

IEEE Documents:

IEEE Std. 141-1993, "Recommended Practice for Electrical Power Distribution for Industrial Plants" (IEEE Red Book).

IEEE Std. 142-1991, "Recommended Practice for Grounding of Industrial and Commercial Power Systems" (IEEE Green Book).

IEEE Std. 242-1986, "Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems" (IEEE Buff Book).

IEEE Std. 1100-1992, "Recommended Practice for Powering and Grounding Sensitive Electronic Equipment" (IEEE Emerald Book).

For copies, call (800) 678-4333.