Keeping the Jib Out of Trouble: How Zone Limiting Draws the Lines a Crane Must Not Cross

▌  APPLICATION NOTE  ·  OCTOBER 2026  ·  ZONE LIMITING WITH LINE, TRIANGLE, QUADRILATERAL AND CIRCLE ZONES

Keeping the Jib Out of Trouble: How Zone Limiting Draws the Lines a Crane Must Not Cross

A tower crane can reach a great deal of ground, and not all of it is safe or permitted. A power line crosses the corner of the site, a fixed structure sits just outside the gate, a neighbouring crane occupies part of the slew, and a pedestrian route runs under the jib. The operator can see the load, but the machine has no idea that a particular patch of ground is out of bounds. Zone limiting gives the black box that idea, turning the note in the lift plan into a boundary the crane itself understands.

This application note looks at the zone limiting function of the ZNAO-TG2 tower crane safety monitoring system. The system supports line, triangle, quadrilateral and circle obstacle zones, and one crane can carry up to eight zones at once. The sections that follow explain what a zone is for, how the four shapes differ, how a site sets the zones and what changes on the deck once they are live. Zone limiting supports the coordination that standards such as ISO 12480-1, EN 14439 and BS 7121 already require; it is a backstop, not a substitute for the lift plan.

01  WHY A CRANE NEEDS A FENCE

On the ZNAO-TG2, zone limiting is one of the core functions, alongside load and moment monitoring and group anti-collision. The point of a zone is to encode site knowledge that no single instrument on the crane could measure. The crane knows its radius, height and slew, and it knows these precisely: radius and height to 0.1 m, slew angle to 0.1 degrees, load to 0.01 percent and wind speed to 0.1 m per second. What it cannot know by itself is that a corner of the site holds a hazard. The zone supplies that meaning.

Once the meaning is in the box, the machine can reason about it continuously. As the trolley travels and the jib slews, the system compares the moving position against the zones it has been given, and it responds when the crane approaches a boundary. This is the everyday value of the function. A zone keeps working while the operator is watching the load, while the signalman is giving a signal, and while the supervisor is looking elsewhere. It does not depend on anyone remembering the hazard at the right moment.

▌  SITE KNOWLEDGE  A zone encodes what the lift plan knows but the crane cannot measure, such as a power line or a fixed structure.

▌  A WORKING BOUNDARY  Zone limiting defines both where the crane may work and where it must stay clear, in terms the machine can act on.

▌  PART OF THE BLACK BOX  It sits beside load and moment monitoring and group anti-collision as a core function of the system.

02  FOUR ZONE SHAPES, AND WHEN TO USE EACH

The system supports four obstacle shapes: line, triangle, quadrilateral and circle. The shape is simply the easiest way to describe the area that must be protected. A line suits a long, thin hazard such as a cable run, a fence or the edge of an adjacent structure, where the concern is not a patch of ground but a barrier the trolley must not cross. A circle suits a single compact hazard, such as a mast, a tower or a single pole, where the risk is a radius around one fixed point.

A triangle and a quadrilateral cover the awkward middle ground. Many real hazards are neither a point nor a long line: a wedge of ground beside an existing building, an approach that bends around a plant room, or an area that follows the shape of an adjoining site. A triangle can fence a corner, and a quadrilateral can wrap a rectangle of ground that must stay free. Choosing the right shape is what keeps a zone simple enough for the crew to understand and precise enough to match the hazard.

▌  LINE  A long, thin barrier such as a cable run or a fence, for hazards where the trolley must not cross a boundary.

▌  CIRCLE  A compact hazard such as a mast, tower or pole, protected by a radius around one fixed point.

▌  TRIANGLE AND QUADRILATERAL  The middle ground, fencing a corner or wrapping a rectangle of ground that must stay clear.

03  UP TO EIGHT ZONES ON ONE CRANE

One crane can carry up to eight zones. That figure matters more than it first appears, because a real site rarely has a single hazard. A steel yard might need a zone for the overhead power line, a second for the neighbouring crane, a third for a fixed structure, a fourth for a pedestrian route that must never be overflown, and more besides. The ability to define several zones on one machine means the site is not forced to choose which hazard to leave unprotected.

Eight zones also allow a shape to be used well. A complex hazard can be described by more than one simple zone rather than by one complicated shape, so the boundary stays readable on the display and in the documentation. Because each zone is defined and named during commissioning, the crew can see at a glance which area a response refers to. The limit is high enough to cover the hazards a busy site actually has, while keeping each individual zone simple enough to trust.

▌  EIGHT AT ONCE  One crane can define up to eight zones, so several hazards can be protected on the same machine.

▌  SIMPLE PIECES  A complex hazard can be described with more than one simple zone instead of one complicated shape.

▌  READABLE BOUNDARIES  Each named zone stays clear on the display and in the documentation, so the crew knows what a response means.

04  HOW A SITE SETS THE ZONES

Setting zones is a commissioning task done with the people who know the site. The supervisor, the crane coordinator and the operator work from the lift plan and walk the ground to agree what must be protected. Each hazard becomes a shape: a line along a boundary, a circle around a mast, a triangle in a corner, a quadrilateral over a rectangle of ground. The shape is then entered and named, and the eight-zone budget is spent where the risk is greatest.

Once the zones are in, they should be tested before the site depends on them. A controlled trial lets the operator drive the trolley and the hook toward a known boundary and confirm that the response behaves as expected. The zones are not a substitute for a banksman or a lift plan; they are a backstop that works even when attention is elsewhere. Because nothing on a site stays still, the zones need a review whenever structures, cables or neighbouring operations change.

▌  AGREE WITH THE SITE  Supervisor, coordinator and operator walk the ground and agree what must be protected before the zones are entered.

▌  SPEND THE BUDGET  Each hazard becomes a line, circle, triangle or quadrilateral, and the eight-zone budget goes where risk is greatest.

▌  TEST AND REVIEW  Trial the zones with a controlled approach, and review them whenever structures, cables or neighbours change.

05  LIVING WITH ZONES: WHAT CHANGES DAY TO DAY

For the operator, a zone is a boundary that behaves the same on every shift. Where a lift would push the trolley or the hook into a protected area, the system responds, so the operator learns the shape of the site through the machine as well as through the plan. Zones give a new operator a faster route to a safe lift, because part of the site knowledge that used to live in experienced heads is now held by the black box itself.

For the supervisor, zones make the plan auditable. Because the system logs alarms for audit, inspection and accident back-trace, a zone entry leaves a record, and the record shows whether the boundary held. That turns an abstract plan into a set of events that can be reviewed on the 11.6-inch sunlight-readable industrial display and in the log. Zones do not remove the need for judgement, but they give judgement something firm to rest on at the end of a long shift.

▌  THE SAME BOUNDARY  A zone behaves identically on every shift, so the operator learns the site through the machine as well as the plan.

▌  FASTER TO SAFE  A new operator reaches a safe lift sooner, because site knowledge is held by the system rather than only by experience.

▌  AN AUDITABLE PLAN  Zone entries are logged for audit, inspection and back-trace, so the plan becomes a set of reviewable events.

PROCUREMENT IMPACT

●  If the crane works near power lines, fixed structures or neighbouring cranes, zone limiting is the function that holds the boundary when attention moves elsewhere.

●  Ask which shapes the site needs, because a line, circle, triangle or quadrilateral describes hazards that a single fixed limit cannot.

●  Confirm how many zones one machine can carry, since a busy site often has several hazards rather than one.

●  Make zone setting part of commissioning, with the supervisor, coordinator and operator agreeing the shapes from the lift plan.

●  Plan a review trigger, so zones are revisited whenever structures, cables or neighbouring operations change.

WHAT TO WATCH

●  Watch the shapes. A zone that is too complicated to explain is a zone the crew will not trust.

●  Watch the budget. Eight zones are generous, but they are not unlimited, so the shapes must match the real hazards.

●  Watch the test. A boundary that has never been trialled is a boundary nobody has seen work.

●  Watch for change. New cranes, new cables and new neighbours can make an old zone wrong.

●  Watch the plan. Zones support the banksman and the lift plan, and they do not replace either.

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