English (GB)
12
5.2.3 Required settings of the motor-protective
circuit breaker
For motors with an MP 204 motor protection unit,
Grundfos recommends using a special trip curve with
P-characteristics at a setting of Un times 5 for 1
second.
For cold motors, the tripping time for the motor-
protective circuit breaker must be less than 10
seconds at five times the rated maximum current of
the motor.
For all Grundfos submersible MMS motors, the
maximum start and stop ramp time is 3 seconds
(minimum 30 Hz).
In order to ensure optimum protection of the
submersible motor, set the motor-protective circuit
breaker in accordance with the following guidelines:
1. Set the overload to the rated maximum current of
the motor.
2. Start the motor and let it run for half an hour at
normal performance.
3. Slowly grade down the scale indicator until the
motor trip point is reached.
4. Increase the overload setting by 5 %.
The highest permissible setting is the rated
maximum current of the motor.
For motors wound for star-delta starting, set the
motor-protective circuit breaker as above, but the
maximum setting must be rated maximum current x
0.58.
The highest permissible startup time for star-delta
starting or autotransformer starting is 2 seconds.
5.3 Lightning protection
The installation can be fitted with a special
overvoltage protection device to protect the motor
from voltage surges in the power supply lines if
lightning strikes somewhere in the area. See fig. 7.
Fig. 7 Connecting an overvoltage protection
device
The overvoltage protection device will not, however,
protect the motor against a direct stroke of lightning.
Connect the overvoltage protection device to the
installation as close as possible to the motor and
always in accordance with local regulations. Ask
Grundfos for lightning protection devices.
MS 402 submersible motors, however, require no
further lightning protection as they are highly
insulated.
Ask for a special cable termination kit with a built-in
overvoltage protection device for Grundfos 4"
submersible motors (product no. 799911/799912).
5.4 Cable sizing
Make sure that the submersible drop cable can
withstand permanent submersion in the actual liquid
and at the actual temperature.
The cross-section (q) of the cable must meet the
following requirements:
• The submersible drop cable must be sized to the
rated maximum current of the motor.
• The cross-section must be sufficient to make a
voltage drop over the cable acceptable.
Grundfos supplies submersible drop cables for a
wide range of installations. For correct cable sizing,
Grundfos offers a cable sizing tool.
Fig. 8 Cable sizing tool
The sizing tool provides a accurate calculation of the
voltage drop at a given cross-section on the basis of
the following parameters:
• cable length
• operating voltage
• full-load current
• power factor
• ambient temperature.
If this requirement is not met, the motor
warranty will be invalidated.
TM00 1357 3605
825045 825017
L1
L2
PE
L3
L
PE
N
Submersible motor cables are
dimensioned for submersion in the liquid,
and will not necessarily have sufficient
cross-section to be in air.
TM05 8770 2613