Overload and Threshold Alarms: How Limits and Auto-Alarm Behaviour Work in Daily Practice

▌  APPLICATION NOTE  ·  OCTOBER 2026  ·  OVERLOAD AND THRESHOLD ALARMS

Overload and Threshold Alarms on a Tower Crane: How Limits and Auto-Alarm Behaviour Work in Daily Practice

Every tower crane has limits, and every working day brings the crane close to them. The load chart says what may be lifted at a given radius, the wind limit says when a lift should stop, and the moment limit ties the two together. The operator's job is to stay inside those lines all day, through hundreds of cycles, in weather and light that change hour by hour. An alarm system exists to make that job visible, not to do it for the crew.

The ZNAO-TG2 tower crane safety monitoring system, product code CR-SMS-TG2, watches load weight, moment, trolley radius, hook height, slew angle, wind speed and tilt, and raises overload and threshold alarms with a sounder. This note explains where the limits come from, what the system actually does when a value crosses one, and how crews set thresholds and zones so that alarms stay meaningful instead of becoming background noise.

01  WHERE THE LIMIT COMES FROM

The limits are not invented by the monitoring system. They come from the crane's load chart, from the lift plan and from the standards that govern the site. What the system does is hold those limits as settings and compare them against live measurements, cycle after cycle. The distinction matters at procurement time, because the buyer is choosing a device that enforces the site's own rules, not a device that supplies them.

Because the comparison is continuous, an alarm is not a verdict on a single moment but a statement about the current state of the lift. Load alone does not decide safety: a heavy load near the mast may be well inside the chart, while a light load far out on the jib is not. That is why the system watches moment and radius together with load rather than treating each value in isolation.

▌  LIMITS ARE YOURS  The load chart, the lift plan and local standards set the limits; the monitoring system holds and applies them.

▌  MOMENT, NOT JUST WEIGHT  Load, radius and moment are read together, because a light load far out can be the harder lift.

▌  CONTINUOUS COMPARISON  The system compares live values against its settings through every cycle, not once at the start of a shift.

02  WHAT THE SYSTEM WATCHES AND WHEN IT SOUNDS

The system senses the seven quantities that define a lift and compares each against its setting. When a value crosses a threshold, the overload or threshold alarm is raised with a sounder in the cab, so the warning arrives where the person who can act on it is sitting. The ZNAO-TG2 states radius to 0.1 m, height to 0.1 m, slew to 0.1 degrees, load to 0.01 percent and wind to 0.1 m/s, which is fine enough to warn on the approach to a limit rather than only at the limit itself.

The same host watches its own sensors. If a sensor fails, the system raises a yellow prompt and resets automatically once the fault clears. That behaviour is deliberately different from an overload alarm: a fault is a statement about the instrument, while an overload is a statement about the lift, and a crew must be able to tell the two apart instantly, in two different colours and two different states of the machine.

▌  SOUNDER IN THE CAB  The alarm sounds where the operator sits, so the warning arrives with the person who is moving the load.

▌  FINER THAN THE LIMIT  Load to 0.01 percent and wind to 0.1 m/s let the system warn on approach, not only at the edge.

▌  FAULT VERSUS OVERLOAD  A sensor-failure yellow prompt with auto-reset is kept distinct from an overload, so the two are never confused.

▌  WIND AND TILT  Wind speed and tilt are watched alongside the load values, because conditions can change faster than the cycle.

03  SETTING THRESHOLDS AND ZONES IN PRACTICE

An alarm is only useful if the crew believes it. That is a setting problem as much as a hardware problem. Thresholds set too close to the everyday working values produce a stream of warnings that people learn to ignore, and a warning that is ignored is worse than no warning at all. Thresholds set too loosely fail to warn in time. The practical answer is to set them against the load chart and the lift plan, then review the settings whenever the job changes.

Zone limiting gives the system a second way to warn. The ZNAO-TG2 supports line, triangle, quadrilateral and circle obstacle zones, up to 8 zones, so a crane working near a power line, a neighbouring structure or a public area can be told where not to go. Group anti-collision adds multi-crane auto-networking with relative-coordinate calculation and pre-warning, which is the tower crane equivalent of keeping an eye on the crane beside you.

▌  BELIEVABLE THRESHOLDS  Settings that fire constantly train people to ignore alarms, so thresholds must match the load chart and the job.

▌  ZONES AS BOUNDARIES  Line, triangle, quadrilateral and circle zones, up to 8 of them, define where the crane must not travel.

▌  GROUP ANTI-COLLISION  Multi-crane auto-networking with relative-coordinate calculation gives a pre-warning before two cranes converge.

▌  REVIEW AS THE JOB MOVES  Thresholds set for one phase of a project need revisiting when the crane is tied in or the geometry changes.

04  THE MORNING ROUTINE AND THE ALARM

The alarm only earns its place if the crew checks the system before the first lift. The cab display should be alive, the values should move as the crane moves, and the settings should match the plan for the day. A quick functional check each morning is the difference between a monitoring system and a decoration.

During the shift, the crew should treat an alarm as information rather than as an accusation. The honest response is to stop, read what the system is reporting, and decide whether the lift has genuinely changed or whether a setting needs adjusting. Over time this produces something valuable: a crew that can describe the way the crane behaves, load by load and cycle by cycle, because the system has made that behaviour visible to everyone in the cab.

▌  PRE-LIFT CHECK  Confirm power, display and live values before the first lift, because a system that is not checked is not trusted.

▌  ALARM AS INFORMATION  An alarm prompts a pause and a reading of the numbers, not a reflex defensiveness.

▌  SHARED AWARENESS  Live values on the 11.6-inch 1920x1080 display let the operator and anyone in the cab read the same picture.

▌  RATED FOR THE SITE  IP67 sensors, minus 20 to 60 degrees Celsius and 5 to 95 percent relative humidity keep the system working through the shift.

05  WHAT ALARMS DO NOT DO

An alarm does not lift for the operator, and it does not replace the load chart, the lift plan, the risk assessment or the competence of the crew. It brings a limit into view at the moment the limit matters. The requirement to respect that limit comes from the standards that apply in the market, such as ISO 12480-1, EN 14439, BS 7121, LOLER 1998 or OSHA 29 CFR 1926 Subpart CC, and from the site's own procedures.

An alarm also does not tell the whole story of a lift. It reports a value crossing a line, and the reason may be entirely ordinary: a heavier than expected component, a longer radius as the trolley travels out, a gust passing through. The system is a source of information for a competent crew, not an authority standing above them, and it works best when it is set up with the crew rather than imposed on them.

▌  AN AID TO COMPETENCE  The alarm makes a limit visible; it does not replace the load chart, the plan or the crew's judgement.

▌  CONTEXT MATTERS  A crossing has a reason, and a competent crew reads the reason as well as the warning.

▌  SET UP WITH THE CREW  Systems tuned with the operators are trusted; systems imposed on them are resented and ignored.

PROCUREMENT IMPACT

●  Ask how thresholds and zones are configured, and who can change them, because a system that cannot be tuned to the load chart will eventually be ignored.

●  Confirm that alarm behaviour distinguishes an overload from a sensor fault, so that the crew can respond correctly and quickly.

●  Check the stated resolutions, radius 0.1 m, height 0.1 m, slew 0.1 degrees, load 0.01 percent and wind 0.1 m/s, against the lifts you run.

●  Plan for group anti-collision if your sites have cranes working near each other, since multi-crane auto-networking is a different scope from a single machine.

●  Consider the display and the sounder together, because a warning that is not heard in the cab is a warning that did not happen.

WHAT TO WATCH

●  Watch the rate of alarms in the first weeks after commissioning, because a high rate usually means the settings, not the crew, need attention.

●  Watch the wind alarm in particular, since wind is the value that changes fastest and the one most often argued about.

●  Watch whether the pre-lift check becomes a formality; the morning check is what keeps the system honest.

●  Watch for patterns that repeat at the same stage of a cycle, because repetition is the signal to change a procedure.

●  Watch the local standard that applies to your site and keep the alarm settings consistent with it.

ABOUT  ZN Industrial (Shandong Zhinuo) designs and manufactures crane safety monitoring systems, positioning systems and mechanical auxiliary handling equipment for overseas construction, industrial and steel markets.

Standards and regulations vary by market — local rules still govern your site.

▌  CONTACT  shandongzhinuo@outlook.com  +86 176 6371 0314 · en.znaqkj.com

SOURCES · ISO 12480-1 · EN 14439 · BS 7121 · LOLER 1998 · OSHA 29 CFR 1926 Subpart CC