FEATURE ADVERTISEMENT FEATURE
Effective and 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
A
n unearthed (or IT) system as per BS7671 section 3 and IEC60364-4, unlike a TN system 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 circuit as it can lead to inadvertent switching or false tripping without detection. See double high resistance symmetrical insulation fault on DC PLC circuit below, 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 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.
TYPICAL STRUCTURE OF INSULATION MONITORING DEVICES IN A CONTROL SYSTEM This diagram 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.
Bender UK
www.bender-uk.com T: 01229 480 123 Typical structure of Insulation Monitoring Devices in a Control System.
TN SYSTEMS: Neutral conductor N and ground conductor PE are connected via bonding jumper. First low R earth fault results in disconnection of supply.
IT SYSTEMS: The ungrounded system has no direct connection between live conductors and earth. First low R earth fault results in continuity of operation.
18 AUGUST/SEPTEMBER 2018 | IRISH MANUFACTURING / IRISHMANUFACTURING
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