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EMC and RCDsTo the Editor: I am an electrical engineer with a manufacturer of large food-processing equipment. A safety issue has surfaced at our international office (Leicester, UK), and I am wondering if any of your readers have heard of this particular problem and if they know of any solution that the European committees may have adopted and published. We are having problems with our products that utilize an electronic variable frequency drive (inverter). The equipment is 400 V, three-phase, 7.5 hp through 5 hp. Due to the EMC Directive, we need to use a three-phase power line filter to meet the specifications. The problem is that when our customers turn on the main power disconnect (isolator) on our equipment, it trips their residential current devices (RCDs). Please note that if the RCD doesn't trip when the isolator is turned on, the machine runs just fine. If the load is balanced, there is only 0.1–0.2 mA of leakage current. The problem has been isolated to the power line filters that have capacitors going to ground, which, at switch-on, result in high transient currents. The RCDs that are tripping have a sensitivity rating of 30 mA. These 30-mA RCDs are protecting socket outlets located throughout the processing plant into which our equipment may be plugged. I should mention that it is common for our customers to mount our equipment on casters and use portable cords so the equipment can be moved around for cleaning and maintenance purposes. The EMC standards that we are currently testing to are EN 55014:1993 (CISPR 14. 1993-01, Third Edition) and EN 55104 (harmonized September 16, 1995), using EN 61000-4-2, EN 61000-4-4, IEC 1000-4-3, EN 61000-4-5, and EN 61000-4-11. Different component manufacturers have told us we need to utilize a filter with a lower-rated leakage current (this means smaller capacitors to ground). One article recommended the use of higher-rated sensitivity units and time-delay units. Based on our problem, it seems there may be some conflict between the EMC Directive and the use of RCDs. The EMC Directive applies more to the equipment side of things, and the use of 30-mA RCDs is dictated more by electrical installations for buildings. I realize that the ultimate goal of all the standards involved is safety and the protection of personnel, but at this time in Europe I don't know if there's a clear-cut answer. If anyone has any information that would help us solve this problem, I would appreciate it. Ron Bocian |
