Hi Chris:
We use similar circuits.
The circuit is an inverter powered from SELV. The
inverter output is hazardous voltage. Usually, the
output is floating, but not always.
The output MAY be hazardous energy ONLY IF the SELV
is hazardous energy. If the SELV is hazardous energy,
the inverter circuit may provide either hazardous
energy or non-hazardous energy output. If the SELV
is not hazardous energy, then the inverter output is
not hazardous energy (applying the conservation of
energy theorem).
While the output is hazardous voltage, it may not be
hazardous current, i.e., it may be a limited current
circuit. A limited current circuit is one where the
maximum current does not exceed 0.5 mA peak times the
frequency in kilohertz. Since you are at 60 kHz,
then the maximum current into a 2 kohm resistor is
30 mA peak, under both normal and single-fault
conditions. Most faults in the inverter circuit
cause it to shut down, so the output is zero, and
the circuit qualifies as a limited current circuit.
Even if the circuit does not shut down under single-
fault conditions, the impedance of the transformer
is often so high that the output remains as limited
current.
The advantage of the limited current circuit is that
it is considered the equivalent of SELV, and you do
not need Basic, Double, or Reinforced insulation
between the circuit and the body of an operator.
Also, you do not need an electrical enclosure.
If the circuit is not a limited current circuit,
then you must apply the rules for Basic, Double, or
Reinforced insulation.
If the circuit is floating, then any one fault in
Basic Insulation will not cause a shock hazard.
This creates an interesting conundrum in evaluating
the insulation required for protection.
The inverter transformer is subject to
- normal temperature tests (for Class A insulation);
- output short-circuit tests (max temperture or
the cheesecloth test).
Since the transformer is not an isolating transformer,
there is no hi-pot test.
As for start-up voltage, you should determine whether
the output is limited current during the start-up. If
it is, then there are no further requirements.
Standards experts are still considering the problem of
start-up conditions in general. There are no guide-
lines at the moment. All we can say is that if your
circuit complies during startup, then there are no
further requirements. If you design for safety during
startup conditions, then you have an acceptable
construction. If you have hazardous conditions during
startup, and non-hazardous conditions during run, then
you will need to negotiate with your certification
house.
Some warnings...
The HV, HF output easily creates arcs. These arcs can
lead to tracking across the surface of insulators,
including printed wiring boards, which can then lead
to ignition. Test for tracking using a small hand-
held screwdrive drawn across the board from the HV
trace to a ground or LV trace.
Also, while the HV is limited current and is therefore
not a shock hazard, the HV and HF combine to arc to
the skin. This creates a deep burn that takes a long
time to heal. So, stay away from the HV.
Enjoy! And best regards,
Rich
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