INFRASTRUCTURE
In a medical IT/IPS system, an earth
fault does not cause the system to shut down. There is no conductive connection between active conductors and the protective earthing conductor within the IT system. When a fault occurs in an earthed system (TN system), the fuse blows and the supply is automatically disconnected. High electrical energy can occur at the point of the fault creating a potential fire risk. A medical IT/IPS system therefore
meets four essential demands: l when a first insulation fault occurs, the power supply is not interrupted by the tripping of a protective device
l medical equipment continues to function
l fault currents are reduced to a non- critical level for patients and staff
l there is no panic in the operating theatre or intensive care unit (ICU) because there is no power failure.
The medical IT system consists of an isolating transformer, a device to monitor insulation resistance, transformer load and temperature, and a remote alarm indicator installed in the operating theatre or at a manned nurse station nearby. Continuous insulation monitoring
ensures that a deterioration in insulation resistance is immediately detected and signalled, but the power supply is not interrupted and continuity of operation is guaranteed. The insulation monitoring device is a vital element of the IT system that ensures power remains available. Connected between system and earth, it continuously monitors insulation resistance, precisely recording and indicating any faults. It also monitors the load current and
temperature of the transformer to maintain a constant check on the health of the critical electrical supply. The load of an IT system transformer is not infinite. Therefore, any overload of the transformer and indicative change in temperature must be monitored. This means an overload of the system can be signalled and staff informed via the alarm panel, enabling them to respond to the fault by switching off unnecessary equipment, thus reducing the load. The transformer will only shut down to protect against short circuits. An overload does not lead to power failure and does not threaten the continued operation of vital medical equipment.
Uninterruptible power supply The majority of hospitals and medical facilities have back-up generators on site, but critical medical locations - group two and categories four and five - require a uninterruptible power supply (UPS) unit. A UPS provides an emergency power
supply to specified critical loads in the event of mains power failure with an
IFHE DIGEST 2019
How the incorporation of an ATICS within individual IPS units in combination with N+1 stand alone UPS provides increased resilience over traditional parallel N+1 UPS and IPS arrangements.
immediate changeover to an alternative power source when required. UPS differs from an auxiliary or emergency power system or standby generator because it provides near instantaneous protection from input power interruptions, immediately restoring power with energy stored in batteries. The level of protection depends on
specific requirements with battery systems able to maintain power over periods from five minutes to three hours. Suppliers should be able to offer medical UPS from 500 VA up to 800 kVA. Regulations guidance on the use of UPS in UK medical locations is provided by: British Standard BS7671:2008 (2011); HTM 06-01 2017; and ETCI ET101:2008 UPS Battery Systems for Medical Locations The regulations and guidance documents state that in the event of a failure in a group two, category four or five medical location, the battery system for tertiary power supplies should provide an autonomous supply for at least three hours. If there is a secondary power source available with a 25 second change over time, this autonomy can be reduced to one hour. The guidance document HTM 06-01
2017, states that the battery system must have a 10 year design life, thread insert connections, a fire retardant case and provide 60 minute autonomy. There are many ways to configure a
UPS system to ensure the highest level of resilience and protection is achieved. The most common configuration is two UPS systems in a parallel configuration; this is
referred to as a N+1 system and would be in line with the main supply. The N+1 configuration allows maintenance to be completed without disruption to the load. With the ever changing need for power
protection, it is impossible to predict future requirements, so a comprehensive range of modular UPS solutions are now available that ensure the full level of power protection and the flexibility to increase protection without having to redesign or substantially change the infrastructure. Modular UPS units allow a healthcare facility to scale and grow protection as the requirement grows. Using an automatic changeover device with a N+1 UPS system ensures that a medical location benefits from the highest possible level of resilience. The ATICS changeover device has been designed for medical applications and combines an insulation monitor and signal generator for advance earth fault location with a rotary cam changeover device. Together they create an integrated compact system that can be incorporated into a medical IT system without taking up extra space. Where parallel N+1 UPS systems are
combined with single supply IPS systems, there remains a single point of failure on the common output of the UPS. The failure of this output cable would result in the loss of all connected IPS circuits. The ATICS device allows individual IPS systems to be supplied from two diverse sources and removes the single point of failure associated with the traditional single supply cable. In the event of primary supply power loss, the ATICS
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