2.4.4
Low-Voltage Motor Overload Relays
**************************************************************************
NOTE: In most cases, thermal relays will be
specified. Standard, slow, and quick-acting trip
units are available. Magnetic relays are used to
protect low-voltage motors with long starting times
or unusual duty cycles.
**************************************************************************
2.4.4.1
General
[Thermal] [and] [magnetic current] overload relays shall conform to NEMA
ICS 2 and UL 508. Overload protection shall be provided either integral
with the motor or controller, and shall be rated in accordance with the
requirements of NFPA 70. [Standard units shall be used for motor starting
times up to 7 second.] [Slow units shall be used for motor starting times
from 8 to 12 seconds.] [Quick trip units shall be used on hermetically
sealed, submersible pumps, and similar motors.]
2.4.4.2
Construction
Manual reset type thermal relays shall be [melting alloy] [bimetallic]
construction. Automatic reset type relays shall be bimetallic
construction. Magnetic current relays shall consist of a contact mechanism
and a dash pot mounted on a common frame.
2.4.4.3
Ratings
Voltage ratings shall be not less than the applicable circuit voltage.
Trip current ratings shall be established by selection of the replaceable
overload device and shall not be adjustable. Where the controller is
remotely-located or difficult to reach, an automatic reset, non-compensated
overload relay shall be provided. Manual reset overload relays shall be
provided otherwise, and at all locations where automatic starting is
provided. Where the motor is located in a constant ambient temperature,
and the thermal device is located in an ambient temperature that regularly
varies by more than minus 10 degrees C, 14 degrees F, an ambient
temperature-compensated overload relay shall be provided.
2.4.5
Automatic Control Devices
2.4.5.1
Direct Control
Automatic control devices (such as thermostats, float or pressure switches)
which control the starting and stopping of motors directly shall be
designed for that purpose and have an adequate kilowatt horsepowerrating.
2.4.5.2
Pilot-Relay Control
Where the automatic-control device does not have such a rating, a magnetic
starter shall be used, with the automatic-control device actuating the
pilot-control circuit.
2.4.5.3
Manual/Automatic Selection
a.
Where combination manual and automatic control is specified and
the automatic-control device actuates the pilot control circuit of
a magnetic starter, the magnetic starter shall be provided with a
SECTION 16475A
Page 12
Previous Page
