Power Protection Unit (EU Only)

The PPU is fitted with three distinct devices, namely a Residual Current Device (RCD), a Surge Arrestor (SA) and an Inrush Current Limiter (ICL).

The RCD protects users of the Rack against electrical faults, such as shorts caused by damaged cables. It does this by monitoring the incoming current and comparing it against the outgoing current. Any imbalance greater than 30 mA causes the RCD to trip and cut the power to the Rack within 300 ms.

The SA is a device that quenches or dampens high-voltage spikes that may be present on the incoming mains power supply. This can include lightning strikes or electrical noise from nearby equipment such as heavy machinery. The SA includes several devices that combine to provide comprehensive protection. As the SA is designed to absorb unwanted energy, it is subjected to wear and tear, which is displayed on the device as a change in color (see Troubleshooting for details about maintenance schedules and activities). The display starts as green and slowly changes to red as the internal components wear out.

The ICL is designed to protect the incoming mains supply from excessive current draw, which can occur when a large number of power supplies are switched on at the same time. In the case of Rack Model 2, there could be up to thirteen power supplies. The ICL controls this by inserting a large wattage resistor into the circuit for 0.5 seconds, thereafter the resistor is bypassed using a relay. By slowing down the inrush current, this device prevents unwanted tripping of the building’s electrical protection devices.