If you are living in a house in Australia built since some point after the year 2000, your home is almost certainly being protected by a safety switch – normally an RCD or Residual Current Device. The job of the RCD is to cut off power if there is an imbalance between the Active and Neutral conductors, normally caused by a short to ground. The good news is that the RCD is keeping your home safe, protecting your family and making it 40% less likely your home will burn down due to fire. The RCD in my own house has tripped occasionally, normally because a household kitchen device like an electric frypan or the element in my clothes dryer has ‘blown up’ but occasionally it trips for reasons unknown.
I had always presumed that RCDs did not have a role to play in the data centre, but I recently learned that this is not the case. I attended a conference entitled “Meet the Data Centre Experts” where David Morley, Data Centre Engineer from CITEC in Brisbane explained that many of their data centre racks are now protected by RCDs. This change was made after the release of the AS/NZS 3000: 2007 Wiring Rules & subsequent revisions. These revisions specify requirements for Residual Current Protection to be installed to practically all new and modified power circuits; this includes those within the Data Centre.
The only racks at CITEC that do not have RCDs are those where power consumption exceeds the capability of the RCD or where the equipment in the rack is so sensitive (due to its role in say ‘Emergency Services’) that the equipment is quarantined from RCD use.
Because power supply filters create a lot of transient noise and leakage an improved version of the RCD known as a Super immunised RCD is being used to eliminate nuisance trips. They test their RCDs every 6-24 months which means a rolling series of power interruptions on one ‘side’ of each rack. Rather than do rail1 followed by rail2, they normally test all the rail1s in a series of racks and then all the rail2s. This gives a ‘decent interval’ between power supply interruptions so the affected equipment can reset/recover from a power input loss error condition.
Interestingly the main issue that has come out of this is the human factor. Pre-checks occasionally find mis-cabled racks where data centre equipment either has only one power supply or where the equipment has dual power, both cords are attached to the same rail. Even racks that were previously ‘compliant’ can be found to be out of compliance when new equipment is added or worse, when existing equipment is maintained or repaired and the vendor doing the work does not correctly reconnect the power cords. Finally both power supplies need to be operational, so failed power supplies are also unearthed in this process.
The RCDs in use at CITEC appear to come from Geist who coincidentally sponsored the presentation.