FEATURE SURGE & CIRCUIT PROTECTION
Effective control and auxiliary circuit monitoring can reduce downtime and increase profitability
Early identification of earth faults within mission critical AC or DC control and auxiliary supplies can increase profitability by reducing unscheduled shutdowns and the downtime of valuable assets
IT (UNEARTHED) SYSTEM Unlike a TN system, an unearthed (or IT) system as per BS7671 section 3 and IEC60364-4, is tolerant to first earth fault and will not cut power, which makes it more reliable and safer for mission critical systems, protecting both production and people. The IT system is therefore used widely in critical applications like Group 2 Medical locations and AC/DC auxiliary circuit applications for protection, control and tripping supply in substations/power stations. There is a special focus on using IT system for auxiliary circuits where high reliability is required, see BS7671 557.3.4.1 Note 2. However, the full benefit of an IT system can only be realised by detecting development of first earth fault and repairing it before it develops to a second earth fault on a different conductor. The device utilised is called insulation monitoring device (IMD) and further supplemented by fault location system (iFLS) to locate faulty circuit.
INSULATION MONITORING DEVICE (IMD) IN UNEARTHED/IT SYSTEMS BS7671 clause 411.6.3.1 states: “Where an IT system is used for reasons of continuity of supply, an insulation monitoring device shall be provided...” Governing standard of the Insulation monitoring device (IMD) is BS EN 61557-8 as per IEC 60364-4 and BS7671 557.3.4.3. and 538.1.1. BS EN 61577-8 prohibits use of passive devices in an IT circuits, which cannot detect symmetrical faults. A symmetrical fault is particularly dangerous to control
20 JULY/AUGUST 2018 | ELECTRICAL ENGINEERING
Fig. 1 (TN system): The ungrounded system has no direct connection between live conductors and earth. The first low R earth fault results in continuity of operation
circuit as it can lead to inadvertent switching or false tripping without detection. Fig. 3 shows double high resistance symmetrical insulation fault on DC PLC circuit, not detectable by passive device and can cause switch to be seen as closed. BS7671 clause 557.3.201 states: “A control circuit shall be designed, arranged and protected to limit dangers resulting from a fault between the control circuit and other conductive parts liable to cause malfunction (e.g. inadvertent operation) of the controlled equipment.” IMDs must be labelled to highlight system of use i.e AC, DC or AC/DC. Insulation monitoring technology combined with earth fault location systems developed by Bender are
Fig. 2 (IT system): Neutral conductor N and ground conductor PE are connected via bonding jumper. The first low R earth fault results in disconnection of supply
Fig. 5 Fig. 4
Fig. 3
employed in a wide range of critical control circuit applications within
industry sectors that simply cannot afford unscheduled shutdowns or interruptions in production and process operations. The diagram in fig. 4 shows a typical structure of Insulation monitoring devices with fixed and portable earth fault location to precisely identity faults from distribution to control circuit level. Bender technology can be permanently retrofitted and also has been extensively used to locate long standing earth faults via portable terminals. The comparison of BA 300 System with IMD - BA300 battery management system provides following monitoring functions: - Over and under voltage conditions - High impedance alarm - Earth fault alarm - Battery ability to provide load current - Battery in service monitoring Earth fault alarm is an optional function on BA300 and should not be included with an external earth fault monitoring system. Some comparison of earth fault detection features on an IT system is shown in Fig. 5.
Bender T: 01229 480123
www.bender-uk.com
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