Before the First Start: Earthing and EMC Verification for Bosch Drive Retrofits
Photo: Unknown photographer, commissioned by the Institution of Electrical Engineers, Public domain, via Wikimedia Commons
Retrofitting a Bosch drive into an existing UK panel is rarely a clean-sheet exercise. The machine may have been designed for a TN-S supply, the building may now be TN-C-S, and the site may include TT-derived islands fed from local transformers. The drive is only one part of the protective and EMC system. Before the first start, verify the supply earthing arrangement, residual current protection, protective conductors and high-frequency bonding as a single commissioning package.
Identify the supply arrangement at the origin
Start at the intake, not at the drive. Confirm whether the installation is TN-S, TN-C-S or TT from the distributor's records, the main earthing terminal and a site test. In TN-S, protective and neutral functions are separate throughout. In TN-C-S, the combined PEN is split at the origin and the installation's protective conductor must not be recombined downstream. In TT, the installation has a local earth electrode and the fault path depends on an RCD. Record the arrangement on the panel schedule and the drive commissioning sheet; a retrofit often inherits a mixture after extensions.
Check that the drive's EMC filter and surge protection are suitable for that arrangement. Some filters have specific instructions for TT or IT supplies. Do not assume that a filter supplied for one earthing system can be left in circuit on another.
RCD selection and leakage current
Bosch drives, like other variable-speed drives, produce earth leakage current through EMC filter capacitors and motor cable capacitance. The magnitude depends on cable length, motor size, switching frequency and filter configuration. A 30 mA RCD may be inappropriate for a large drive, and a Type AC device may not respond to DC residual current. Select the RCD type and rating from the drive documentation and the installation design, not from the nearest stock item. Type A or Type B devices are commonly required, with Type B or B+ where DC-sensitive detection is needed.
Where RCD protection is required, calculate the expected leakage for normal operation and for the whole circuit. Use a dedicated circuit or a time-delayed RCD for selectivity, and test at half-rated and rated residual current. Never defeat an RCD to stop nuisance tripping. If trips occur, find the cause: long motor cables, multiple drives on one RCD, wet motor windings, or a filter configuration that does not match the supply.
Protective conductors and equipotential bonding
Size the protective conductor to the current edition of BS 7671 and verify it by the adiabatic equation where fault current and disconnection time demand it. The common table minimum is only a starting point; the PE must withstand the let-through energy of the protective device. Do not use the neutral as a protective conductor on the load side of a TN-C-S split.
Bond the drive enclosure, motor frame and machine frame to the same earthing system with low-impedance connections. Use braided straps or purpose-made EMC clamps for high-frequency bonding, not long thin wires. Supplementary bonding may be needed where simultaneous contact is possible between separately earthed metalwork. Keep bonding conductors short and direct.
Terminate the motor cable shield correctly
The motor cable is the most common EMC weak point in a retrofit. Use a shielded cable and terminate the shield at both ends. At the drive, clamp the shield to the enclosure with a 360-degree EMC gland or screen clamp. At the motor, use an EMC gland or a clamp that bonds the shield to the motor frame. Avoid pigtails: a short wire from shield to PE looks neat but raises high-frequency impedance.
Carry the shield through terminal boxes without interruption. Separate motor cables from encoder, analogue and communications cables, and cross at right angles where they must meet. Follow the drive manufacturer's maximum cable length and recommended cable type. If the original motor cable is unshielded, replace it or accept that the EMC performance will be unpredictable.
Commissioning tests that close the loop
Before energising, complete a visual inspection and protective conductor continuity test. Isolate and disconnect the drive before insulation resistance testing; otherwise the test voltage can damage the drive. Test the motor and cable separately, record the values and compare with previous records. For TT installations, measure the earth electrode resistance and verify the RCD disconnection time. For TN systems, measure the earth fault loop impedance at the drive supply.
After energising, check the drive's earth leakage current at no load and at operating speed. Run the motor at low speed first and observe the encoder, analogue signals and communications for noise. Confirm that the anti-condensation heater, if fitted, is interlocked off in run. Label the earthing arrangement and RCD type inside the panel.
Record the results in the maintenance file. The commissioning record should show the supply arrangement, protective device ratings, RCD type, test results and any deviations. That record is what turns a retrofit from a collection of parts into a verified installation.
