Biomedical Engineering Reference
In-Depth Information
TABLE 3.4
Some Voltages Used for HiPot Testing of Insulation Barriers
HiPot Test Voltages for Reference Voltage
U
(V)
Insulation Type
U
50
50
U
150
150
U
250
250
U
1000
1000
U
10,000
2
U
1000
U
2000
Basic
500
1000
1500
2
U
2000
U
3000
Supplementary
500
2000
2500
2(2
U
1500)
2(
U
2500)
Reinforced
500
3000
4000
connections and a point that ties all nonisolated I/O lines and the hot, neutral, and ground
connections of the power cord.
It must be noted, however, that despite its convenience, this method is not always
accepted by regulatory bodies as a reasonable substitute for the tests speci
ed by the stan-
dards. In any case, make sure that the hiPot tester that you use can detect the precata-
strophic breakdowns, since otherwise, the insulation in your instrument may break down,
delivering an instantaneous lethal level of current.
HiPot testing should be conducted after the equipment is preconditioned in a humidity
cabinet. As before, all access covers that can be removed without the use of a tool must be
detached. Humidity-sensitive components which in themselves do not contribute signi
fi
cantly
to the risk of electrocution may also be removed. In addition, however, voltage-limiting
devices (e.g., spark-gap transient-voltage suppressors, Iso-Switches) in parallel with insula-
tion to be tested can be removed if the test voltage would make them become operative dur-
ing the hiPot.
HiPot tests of the various insulation methods must be conducted with the instrument in
a humidity cabinet. For each test the voltage should be increased slowly from zero to the
target potential over a 10-second period and then kept at the required test level for 1 minute.
If breakdown does not occur, tripping the hiPot tester's automatic shutoff,
fi
ff
, the test is com-
pleted by lowering the voltage back to zero over a 10-second period.
Finally, it must be noted that the standards do not except battery-powered equipment
from hiPot testing. Instead, the reference voltage
U
is set to be 250 V. Fully or partially non-
conductive enclosures are not excluded either. In these cases, the same metal-foil method
used for current leakage testing must be used, being careful that
fl
ashover does not occur at
the edges of the foil at very high hiPot test levels.
Most of the circuitry inside commercial hipot testers is really used to control high-volt-
age source and detecting currents that exceed a set threshold. If the $1000 or so needed to
buy a low-end hipot tester are outside your budget, you may conduct design-time tests
using the circuit shown in Figure 3.29. Here, variac T1 is used to change the supply volt-
age to the primary of a high-voltage transformer T2. We used a surplus transformer with a
ground-referenced secondary rated for a maximum output of 5 kV at 5 mA. Not any high-
voltage transformer should be used for this application. A unit with good voltage regula-
tion is needed. Avoid using neon-light transformers because these are built to provide a
constant current to the load. Under unloaded conditions, they will present a voltage of 9 to
15 kV at the load.
The high voltage applied to the device under test starts as 117 V ac. This voltage is con-
trolled using a Crydom solid-state relay. If the AC_ENABLE signal is a logic high,
12 V
dc is applied to the control inputs of the Crydom solid-state relay, and if SW2, the ARM
key switch, is in the on position, 117 V ac will be applied to the inputs of variac T2. T1, a
Magnatek-Triad step-up transformer, is fed directly by the variac's output. The high volt-
age produced by T2 is then applied to the device under test through a touch-proof banana
connector as shown in Figure 3.30.
The current-trip circuit of Figure 3.31 acts as a milliammeter-controlled relay that dis-
connects the supply to the variac if the isolation barrier under test should fail. Ac current
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