I-DRIVE TECHNICAL MANUAL – INSTALLATION PG DRIVES TECHNOLOGY
11.2.4 Ensuring Repeatability
Various precautions should be taken to ensure repeatable protection from ESD. Connections to ESD drain wires etc. should not be
made through painted surfaces. The routing of drain wires must be consistent. Any significant modifications to components or
routings should be tested for its effect on ESD performance.
11.2.5 Drain Path Components
There are various options on providing drain paths for ESD.
Drain Wires
These components are not suitable for use in ESD discharge paths.
Advantages – low cost, low impedance, maximizes protection from ESD by minimizing transients.
Disadvantages – ESD currents can be high and unpleasant if experienced by operators. Connecting to a battery
terminal may compromise safety, as chassis components etc. will become ‘live’. Drain wires may also contribute to
ground loops in some systems or compromise EMC performance.
Resistors
Resistors provide a suitable discharge path for charges occurring or generated between the electrical masses of the system.
Typical values range from 10mΩ to 10kΩ.
Advantages – can limit ESD discharge currents (unless a flashover occurs) to operators etc. Can be used in series with
drain paths to battery ground (0V) terminals as chassis isolation is provided, however, leakage currents must be at a
safe level. Resistors also limit the possibility of creating ground loops or propagating electrical noise (compromising
EMC performance).
Disadvantages – do not provide a low impedance path for ESD so may be ineffective when large ESD discharges
occur.
Varistors
These components provide an effective ESD path where isolation from battery voltages is required. The varistor continuous
voltage rating is normally around twice the battery voltage (1.5 times minimum). Harris varistor type V82Z2 has proven effective in
many 24V and 36V applications.
Advantages – provides circuit isolation and low impedance.
Disadvantages – the low impedance implies that varistors will not protect operators from large discharge currents.
Capacitors & Inductors
These components are not suitable for use in ESD discharge paths. Capacitors tend to offer protection by breaking down and
operating like a varistor; by implication the capacitor is progressively damaged. Inductors have a high impedance to fast
transients. However, ferrite beads may provide some additional protection where other solutions prove marginal.
Spark Gaps
Spark gaps designed into PCBs may provide some very useful, front line defence against large ESD discharges. However, they
need careful design, a low impedance ground connection and their performance will vary with surface finish and
contamination.
SK76977-07 58
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