Limited power sources
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A limited Power Source Shall incorporate an isolating transformer
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A limited Power source shall comply with one of following:
- the output inherently limited in compliance with table 2B; or
- an impendence limits the output in compliance with table 2B. if a PTC is used,
it shall pass the tests specified in IEC 60730-1, clause 15, 17, J15 and
J17; or
- an overcurrent protective device is used and the output is limited in compliance
with table 2C; or
- a regulation network limits the output in compliance with table 2B, both under
normal and after single fault; or
- a regulating network limits the output in compliance with table 2B, and
overcurrent protective device limits the output in compliance with table
2C after any single fault.
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Where an overcurrent protective device is used, it shall be fuse or a
non-adjustable, non-autoreset, electromechanical device.
Table 2B - Limits for inherently limited power sources
Output Voltage (Uoc)
Output Current (Isc)
Apparent Power
(S)
V a.c.
V d.c.
A
≤20
≤20
≤ 8.0
≤ 5*Uoc
20 < Uoc ≤ 30
20 < Uoc ≤ 30
≤ 8.0
≤ 100
-
20 < Uoc ≤ 60
≤ 150/Uoc
≤ 100
VA
1. Uoc: Output Voltage measured in accordance with 1.4.5 with all
load
circuit disconnected Voltage are fir substantially
sinusoidal a.c. and ripple fee d.c. fir non-sinusoidal a.c. and
d.c. with ripple greater than 10% if the peak, the peak voltage
shall not exceed 42.4V
2. Isc: Maximum output current with any non-capacitive load.
including a short circuit. measured 60 s after application of
the load.
3. S(VA): Maximum output VA with any load. initial transients
lasting less than 100 ms are permitted to exceed the limit.
Table 2C - Limits for power sources not inherently limited
(Over current protective device required)
Current rating of
Output Voltage (Uoc)
V a.c.
V d.c.
Output
Apparent
overcurrent
Current (Isc)
Power (S)
protective device
A
VA
(S)
VA
≤20
≤20
≤ 5
20 < Uoc ≤ 30 20 < Uoc ≤ 30 ≤ 1000/Uoc
-
≤ 250
20 < Uoc ≤ 60
≤ 100/Uoc
≤ 100/Uoc
1. Uoc: Output Voltage measured in accordance with 1.4.5 with all
load
circuit disconnected Voltage are fir substantially
sinusoidal a.c. and ripple fee d.c. fir non-sinusoidal a.c. and
d.c. with ripple greater than 10% if the peak, the peak voltage
shall not exceed 42.4V
2. Isc: Maximum output current with any non-capacitive load.
including a short circuit. measured 60 s after application of
the load. Current limiting impedance in the equipment remain
in the circuit during measurement, but overcurrent protective
devices are bypassed
3. S(VA): Maximum output VA with any load. Current limiting
impedances in equipment remain th the circuit during
measurement, but overcurrent protective devices are bypassed
Initial transients lasting less than 100mS are permitted to
exceed the limit.
NOTE: The reason for making measurements with overcurrent
protective devices bypassed is to determine the amount of energy
that is available to cause possible overheating during the
operating time of the overcurrent protective devices.
4. The Current rating of overcurrent protective devices are based
on fuses and circuit breakers that break the circuit within 120s
with a current equal to 210% of the current rating specified
in the table.
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Condition 2, of table 2B and 2C - Isc: measured 60s after application the load.
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Condition 3, of table 2C - S(VA): overcurrent protective devices are by-passed.
Relevant decision of OSM
- Higher voltage are not allowed (Tables 2B and 2C to followed). Behind a limited
power source higher voltage can be created within secondary circuits out of the low
voltage output of the limited power source. However, an assessment has to be made
to ensure that there is no fire or ignition hazard.