Applications such as remote water pumping stations can be subjected to wide temperature changes, direct sunlight and frost which create conditions for condensation on de-energised electrical equipment. Power electronic based converter systems including VSDs can be susceptible to the effects of condensation, leading to moisture contamination of PCBs.
A build-up of dust on insulated surfaces can create a fire hazard - relatively low levels of leakage current - less than 80mA - travelling across an insulated surface can ignite combustible material and fluids. Scintillations on the insulation surface create conductive tracks that result in a cascade effect and increased leakage current and fire hazard.
The risk of an electrical fire increases again if equipment is mounted out of sight and or subject to infrequent maintenance. Where conventional protective bonding cannot be relied on to achieve the required level of safety, and RCD protection is required, only Type B RCDs can be used with three-phase VSDs (see GAMBICA Installation Guide 4th edition).
BS7671 compatibility of electrical equipment
Regulation: 331.1 deals with the requirement to make an assessment of any characteristics of the equipment that impact on the safety of the installation. These ‘harmful effects’ include earth leakage currents, high protective conductor current present under normal operation, DC feedback and high-frequency oscillations (Figure 1). Waiting until the equipment is on site can result in expensive re-design and remedial work.
Drive installations produce operational leakage currents across a wide frequency spectrum (Figure 2). In this example, the installation had to be designed for use with 30mA RCDs to achieve protection at 50Hz. Under normal operating conditions the leakage currents flowing in the protective conductor should not cause unnecessary operation of the RCD. The DFS4 B NK 30mA will offer 30mA protection at 50Hz (the most dangerous frequency for the human heart) but also restrict leakage currents from 1- to 100kHz to 300mA, reducing the risk of electrocution and/or fire from high frequency currents.
The EMC filter - internal or external to the VSD - produces a fixed leakage current to earth, with frequency components between 100Hz to 1kHz plus leakage currents in the resonant frequency range of the filter. Low cost filters can create high levels of leakage current so low leakage current EMC filters are available for installations covered by Regulation 532. Purchasing filters without sufficient technical data is likely to lead to safety problems.
Varying the speed of the motor via the inverter will produce additional leakage current components of at least 1kHz. A screened cable connected to earth acts as a capacitor discharging at the associated frequency. If the switching frequency of the inverter is in the resonant frequency range of the external EMC filter, extremely high leakage currents can flow. Changing the switching frequency may still produce a multiple of the filter resonant frequency and the problem will still be present.
Transient leakage currents occur during switch-on / switch-off and can cause the RCD to trip. Using switching devices with slow break contacts can substantially increase the risk of tripping but can be avoided using switch disconnects with snap action contacts to connect the main supply to the drive installation. Type B RCCBs have a higher resistance to in-rush residual currants with a maximum duration of 10ms.
When there is a requirement to include RCD protection, BS EN 62477-1:2012 ‘Safety requirements for power electronic converter systems and equipment’ recommends the use of Type B RCDs for three-phase VSDs which produce smooth DC fault currents. This recommendation is also repeated in the Gambica Installation Guide.
Doepke offers a free 64page Technical Application Guide covering RCDs and measures for reducing associated leakage currents.
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