Residual current monitoring in data centres

Residual current devices (RCDs) have long had a bad reputation for causing nuisance tripping and unexplained outages in data centres. Sadly, this perception still exists within the ICT industry and is inconsistent with the latest developments in technology.

Residual or leakage current is any current flowing to earth and is generated by all IT power supplies, typically 0.5 to 1.2 mA per connected power supply. Leakage current is normally a by-product of the power supply; however, in cases involving people it is likely to be a situation where contact has been made with an active conductor. A leakage current of just around 30 mA is enough to stop the heart. Often issues occur when hardware or cabling is being modified or due to faults within the equipment, cabling or rack, and typically when personnel are involved. When a person comes into contact with a live conductor it may cause fatal accidents. RCDs provide protection against fire and electric shocks caused by electrical faults. They are designed to trip at a leakage current of 30 mA within 300 ms. Standard commercial Type II RCDs commonly trip when leakage current is as low as 15 mA of current flow well under 30 ms. They offer high level of personal protection but are unreliable in the data centre environment. Data centres present electrical, chemical and mechanical serious safety risks that can cause injuries or even death if they are not properly identified and mitigated. The high-density IT equipment and servers latched on to sliding rails present risks.

Technological advances

The latest development in residual current RCD technology stops nuisance tripping, enables regular testing of the equipment redundancy and greatly improves safety. The new generation of RCDs, commonly referred to as SI (super immunised) or Ai (high immunity), is fast becoming a game changer for leakage current protection in IT environments. They handle high leakage currents with high reliability while still offering excellent protection for equipment, personnel and environments. Racks filled with SAN, network equip and servers, some producing greater than 15 mA of leakage current per circuit, will not experience nuisance tripping with the immunised RCDs. It should be noted that the latest changes to WHS legislation and Australian Standards make these devices mandatory in most situations.

Standards

Australian standards mandate regular testing and tagging of RCDs to ensure they are working effectively. One of the key benefits of this is that it not only tests the RCD, but also the redundancy of the system and all devices within the rack. Many failed power supplies, incorrectly connected devices and single supply devices can be detected during each test period. To effectively use RCDs in ICT environments, equipment should be dual corded/dual power supplied, or be connected to an in-rack automatic transfer switch (ATS).

While several locations are allowed for RCDs under AS3000 standards, locating devices adjacent to the socket outlets greatly eases the testing process. It is possible to locate the RCDs into distribution boards; however, this can use excessive and valuable chassis space. Installing the RCDs into small enclosures at the rack level has proven to be the most economical and flexible solution. However, some manufactures can now build RCDs into the power rails.

The dual or redundant supply nature of the devices is where one of the key considerations in regards to RCD protection and compliance with AS 3000 come into play. The RCD exemption sections in AS3000 are often read out of context for IT equipment, particularly enterprise-class equipment that will utilise redundant power supplies and is not at risk of failing with the tripping of a single RCD. AS3000 also states that RCD protection is required for all socket outlets not exceeding 20 A. Commonly, dual 32 A circuits are provided to racks to avoid installing RCD protection. Typically, power rails bristling with 10 and 15 A socket outlets are then connected to the 32 A circuits. The problem is back again with socket outlets of less than 20 A requiring protection.

Standard exemptions

There has been a lot of talk amongst operators in regards to the exemptions listed in the clauses 2.6.3.2.1 of AS3000; however, there is only one exemption that can be considered within the context of appropriately redundant equipment. “Where the disconnection of a circuit by an RCD could cause a danger greater than earth leakage current.” In essence, this clause is asking for a risk assessment process to exclude RCD protection. The risk of the services being provided from the equipment should be evaluated against the risk of the equipment being isolated due to an RCD nuisance trip. Typically, a lot of operators will cite the following clause to exempt themselves from installing RCDs: “The connected equipment is required by the owner or operator to perform a function that is essential to the performance of the installation and that function would be adversely affected by a loss of supply caused by the RCD operation.”

This would be valid if data centre facilities only contained single-corded equipment. With dual-corded equipment the loss of one supply does not equate to the device not performing its intended function. Almost all modern IT equipment has dual supply capability or can be fitted to in-rack automatic transfer switches (ATSs) to enable the devices with redundant feeds. But what if multiple RCDs nuisance trip for the same rack resulting in both circuits being isolated? If multiple RCDs activate for the same rack (two or more circuits) it would be a genuine fault and the facility has been saved from a dangerous event. It may have stopped the equipment but most likely saved a life or prevented activation of fire suppression systems. Section 4 of the harmonised WHS legislation mandates RCDs to be fitted to all applicable circuits within the workplace. The fines associated with the various WHS and electrical safety authorities across Australia vary across different states.

Image credit: ©iStockphoto.com/Baran Ozdemir