FAQs - Three Phase UPS Systems

Most UPS manufacturers supply a three phase UPS from 10kVA or 10kW upwards. This unit will either be only available as a 3/3 (3phase-input/3-phase output) or a 3/3 and 3/1 (3-phase input/1-phase output) system. When installing a UPS capable of 3/3 or 3/1 configuration, the final set-up is usually made during installation either through hardware or hardware and firmware settings.

Phase imbalance occurs in a 3-phase UPS system when one or more of the three phases has a higher load than the other phases. The unbalanced voltages lead to unbalanced currents and increased temperature on the ‘overloaded’ phases. Long term phase imbalance can lead to component or catastrophic failure of the uninterruptible power supply.

A 3-phase UPS system will have a built-in automatic bypass for overload and fault conditions. The bypass will generally be a static bypass rather than a relay-based bypass. The UPS will also have a built-in maintenance bypass to allow the UPS to be isolated for maintenance by a manufacturer certified engineer. The UPS installation may also have an external wrap-around UPS bypass switch. This allows the uninterruptible power supply to be fully powered down and if necessary swapped-out, without disruption to the connected loads. The external bypass switch may use a rotary switch or breaker arrangement.

A three-phase (3-phase) supply carries power on three live conductors (L1, L2, and L3), plus a neutral and an earth. The alternating current on each conductor is 120 degrees out of phase with the other two, so at any moment at least one phase is near its peak, and the power delivered never drops to zero. In the UK, each phase is 230V to neutral, and the voltage between any two phases is 400V, both at 50Hz. Three-phase is the standard supply for data centres, large commercial buildings, and industrial sites. Smaller offices and homes normally have a single-phase 230 V supply.

A three-phase (3-phase) supply delivers about three times the power of a single-phase (1-phase) supply for the same current per conductor. Three-phase power is calculated as √3 × 400V × current, while single-phase power is 230V × current. At 100A, a single-phase supply delivers about 23kVA and a three-phase supply about 69kVA. This means smaller cables and breakers can carry large loads. Single-phase UPS systems typically top out at around 20kVA because of the cable size needed to carry the current; above that, three-phase is the standard, practical choice. Three-phase UPS systems start at around 10kVA and scale to several megawatts when units or modules are run in parallel.

An external wrap-around UPS maintenance bypass lets you isolate a UPS system completely for maintenance or replacement while the load stays powered from the connected mains power supply. The external bypass panel usually has three switches: UPS input, UPS output and bypass. Interlocking makes sure they can only be operated in the correct order. If the bypass closes while the UPS inverter is still supplying the load, two unsynchronised sources connect, which can damage the UPS and drop the load. Interlocking is done with mechanical key interlocks (such as Castell keys), or with auxiliary contacts wired back to the UPS so it transfers to its internal static bypass before the external bypass closes. The safe sequence is: (1) UPS to internal static bypass, (2) Close the external bypass, (3) Open the UPS output and finally (4) Open the UPS input. To return to UPS operation, reverse these steps. Rotary bypass switches with ‘UPS / Bypass / Test’ positions build this interlocking into the switch itself.

A qualified electrician or electrical contractor should carry out all sizing and electrical work. They will need the maximum input current, usually provided in the UPS manufacturer’s installation manual. The input current is higher than the UPS output current because of energy losses within the UPS AC-DC-AC conversion and also when the UPS battery is recharging. As a rough guide, a 30kVA UPS draws about 43A per phase at full output, rising to roughly 50A while charging the batteries, so a 63A triple-pole and neutral breaker is typically specified.

UPS Manufacturers usually call for Type C or D curve breakers to handle inrush currents. The rectifier input, bypass input, UPS output and battery DC circuit each need their own protection.

Cables for 3-phase UPS system installations can be H07RN-F (Heavy-Duty Flexible Rubber) or XLPE-insulated, steel wire armoured (SWA), in 5-core (L1, L2, L3, N, E) or 4-core plus a separate earth. Low smoke zero halogen (LSZH) sheathing is the usual choice inside occupied buildings and data halls. Size the cable for the breaker rating, installation method, grouping, ambient temperature, voltage drop and fault current. Larger systems often use single-core cables on cable trays.

Firstly, please remember that all work should be designed and installed to BS 7671 by a qualified electrical contractor. They will need to consider phase balance, neutral sizing, electrical discrimination, resilience and supply capacity. Aspects that we would cover during a site survey.

For phase balancing, the goal is to spread the IT load as evenly as possible across the L1, L2 and L3 supplies, either by allocating rack PDUs or circuits to different phases or by using three phase rack PDUs. A heavily loaded phase runs hotter and stresses the UPS.

In terms of neutral sizing, the switch-mode power supplies powering IT servers create harmonic currents that add together in the neutral, so the neutral conductor may need to be the same size as the phase conductors, or larger.

Electrical discrimination requires coordination of the the breakers so a fault on one server rack circuit trips only that circuit, not the UPS output breaker. This is in case of fault (short circuit) or overload condition.

Resilience in terms of electrical supplies concerns using A and B power feeds to dual-cordded servers and the use of a separate or common bypass supply, and coordination of the UPS system with a local standby generator.

Supply capacity refers to the incoming supplies and main LV board and sub-distribution circuits to ensure they can carry the maximum UPS input current.