Safety Limits - Manitowoc Marine Q 1800 Service Manual

Refrigeration System

SAFETY LIMITS

General
In addition to standard safety controls, such as high
pressure cut-out, the control board has two built in
safety limit controls which protect the ice machine
from major component failures.
Safety Limit #1: If the freeze time reaches 60
minutes, the control board automatically initiates a
harvest cycle. If three consecutive 60-minute freeze
cycles occur, the ice machine stops.
Safety Limit #2: If the harvest time reaches 3.5
minutes, the control board automatically returns the
ice machine to the freeze cycle. If three consecutive
3.5 minute harvest cycles occur, the ice machine
stops.
Determining Which Safety Limit
Stopped The Ice Machine
When a safety limit condition causes the ice
machine to stop, the harvest light on the control
board continually flashes on and off. Use the
following procedures to determine which safety
limit has stopped the ice machine.
1. Move the toggle switch to OFF.
2. Move the toggle switch back to ICE.
3. Watch the harvest light. It will flash one or two
times, corresponding to safety limits 1 and 2, to
indicate which safety limit stopped the ice
machine.
After safety limit indication, the ice machine will
restart and run until a safety limit is exceeded again.
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Analyzing Why Safety Limits
May Stop the Ice Machine
According to the refrigeration industry, a high
percentage of compressors fail as a result of
external causes. These can include: flooding or
starving expansion valves, dirty condensers, water
loss to the ice machine, etc. The safety limits
protect the ice machine (primarily the compressor)
from external failures by stopping ice machine
operation before major component damage occurs.
The safety limit system is similar to a high pressure
cut-out control. It stops the ice machine, but does
not tell what is wrong. The service technician must
analyze the system to determine what caused the
high pressure cut-out, or a particular safety limit, to
stop the ice machine.
The safety limits are designed to stop the ice
machine prior to major component failures, most
often a minor problem or something external to the
ice machine. This may be difficult to diagnose, as
many external problems occur intermittently.
Example: An ice machine stops intermittently on
safety limit #1 (long freeze times). The problem
could be a low ambient temperature at night, a
water pressure drop, the water is turned off one
night a week, etc.
When a high pressure cut-out or a safety limit stops
the ice machine, they are doing what they are
supposed to do. That is, stopping the ice machine
before a major component failure occurs.
Refrigeration and electrical component failures may
also trip a safety limit. Eliminate all electrical
components and external causes first. If it appears
that the refrigeration system is causing the problem,
use Manitowoc's Refrigeration System Operational
Analysis Table, along with detailed charts,
checklists, and other references to determine the
cause.
The following checklists are designed to assist the
service technician in analysis. However, because
there are many possible external problems, do not
limit your diagnosis to only the items listed.