Proper philosophy is vital
to the safe design of most control applications. The following checklist
should be used as a guide.
- Checkout. Check interlock chains for proper operation after installation, after
any modification, and during periodic routine testing.
- Fail-safe design. Design all control circuits to be “fail-safe.” Starting with a breaker
or fuse, the circuit should go through all the interlocks in series
to momentary on-off switches that energize and “seal in” a control
relay. Any open circuit or short circuit will de-energize the control
circuit and must be reset by overt act.
- Interlock Bypass
Safeguards. Establish a systematic procedure for temporarily bypassing
interlocks. Follow-up procedure should be included to ensure removal
of the bypass as soon as possible. When many control-circuit points
are available at one location, the bypassing should be made through
the normally open contacts of relays provided for this purpose. In an
emergency, these relays can be opened from a remote control area.
- Isolation. Isolate control power from higher-power circuits by transformers, contactors,
or other means. Control power should be not more than 120V, ac or dc.
All circuits should use the same phase or polarity so that no hazardous
additive voltages are present between control circuits or in any interconnect
system. Control-circuit currents should not exceed 5A.
- Lockout. Use a keyed switch in interlock chains to provide positive control of
circuit use. To ensure power removal before anyone enters the enclosure,
this same key should also be used to gain access to the controlled equipment.
- Motor Control
Circuits Motor circuits must have a positive disconnect within view
of the motor or, if this is not practical, a disconnect that can be
locked open by the person working on these motor circuits.
- Overvoltage Protection. Control and instrumentation circuits used with high-voltage equipment
must have provision for shorting fault-induced high voltages to ground.
High-voltage fuses with a high-current, low-voltage spark gap downstream
from the high-voltage source are recommended. This also applies to all
circuits penetrating high-voltage enclosures.
- Voltage Divider
Protection. The output of voltage dividers used with high voltages
must be protected from overvoltage-to-ground within the high-voltage
area by spark gaps, neon bulbs, or other appropriate means.
- Current Monitors. Measure currents with a shunt that has one side grounded, or with current
transformers that must be either loaded or shorted at all times.
- Instrument Accuracy. Check instrumentation for function and calibration on a routine basis.