A tower crane on an urban site almost never works in open space. The jib passes over a neighbour’s yard, over a pavement, next to a power line or into the working range of another crane. Zone limiting for a crane (zoning) sets in advance where the load will not go, and an anti-collision system is intended to prevent contact between two cranes whose work areas overlap. In this guide you will find the difference between the two, how they work, how to plan heights and coordination on site, and what to check throughout the project.
This guide was reviewed by RMA’s engineering and service team, and is based on day-to-day work with such systems and on guidance from safety bodies in Israel and Europe.
A tower crane is designed to slew 360 degrees without interference. The site itself almost always sets limits. These are the common situations in which prohibited zones are defined:
An important principle emphasised by safety engineers in Israel: the main risk is the load hanging on the hook, not the jib itself. So in many cases the jib without a load is allowed to pass over a road or a yard, and the hook with a load is prevented from entering the area. There is also a safety reason for this: at the end of the working day the jib must be free to slew with the wind, and locking the jib or fully blocking slewing may create a dangerous load on the crane in strong wind.
A zone-limiting system is fitted to a single crane. It continuously reads the state of the crane and compares it to a map of zones defined at installation by a qualified technician. Israel’s tower crane regulations require limit switches on trolley travel, or other devices that limit its travel, but a fixed limit switch restricts the whole working range. A computerised system allows, for example, the trolley to travel to the end on one side and to stop early on the side facing the road.
A long jib does not stop immediately, because of its inertia while slewing. So the system works in two stages. When the crane enters the approach zone, the system reduces the relevant movement to a low speed. When it reaches the edge of the buffer zone, movement toward the prohibited zone is stopped. From there the crane operator can move only in the opposite direction or parallel to the boundary, at low speed, until leaving the zone. The buffer zone is meant to cover what the system does not measure: mast flex, the effect of wind, the size of the load and its swing. In some systems a height can also be defined for a zone, so that the hook can pass over it only when the load is above a certain height. A discharge area right next to the edge of a prohibited zone will make the crane work at low speed for much of the time.
A typical system receives from sensors the slewing angle of the jib, the position of the trolley along the jib (the radius) and the height of the hook. On cranes with a luffing jib, the angle of the jib is also measured. From this data it calculates at every moment the position of the jib and hook relative to the boundaries.
When there are overlapping cranes on a site, each crane gets its own unit, and the units exchange position data over a continuous radio link. Each unit calculates the distance between its jib, counter-jib and hook and the structure and hoist rope of the neighbouring crane. The basic sequence of operation:
A system that links cranes depends on a stable radio link. When the link to another crane is lost, most systems assume the other crane could be anywhere in its range, and block the whole overlap zone. That is safe, but stops work, so it is important to work on approved frequencies and equipment. In Britain and France the guidance refers to licensing a dedicated frequency with the regulator, and in Israel wireless equipment is subject to Ministry of Communications approvals.
When two cranes are next to each other in the overlap zone, the system slows both. Which one continues first is an organisational decision. The order of priority is set in the site procedure, for example priority for the crane serving an active concrete pour, and is conveyed to the crane operators through the coordinating signaller or the lifting coordinator. The Italian guidance also requires the priority to be set in advance and given to the crane operators in writing.
| Topic | Zone limiting (zoning) | Anti-collision |
|---|---|---|
| What it protects against | A load or crane parts entering fixed areas: a road, a neighbour, a public institution, a power line | Contact between cranes: jib, counter-jib and hoist rope |
| How many cranes | One crane against a zone map | Two or more cranes with a shared work area |
| Data | Slewing angle, trolley radius, hook height | The same data, from all cranes, in real time |
| Communication | Not needed between cranes | Continuous radio link between units |
| Response | Slowing in the approach zone and stopping before the buffer zone | Slowing of both cranes and stopping before contact |
| Limitations | Does not know the size of the load or its swing | As with zoning, and also depends on the quality of the radio link |
| When it is needed | A site next to neighbours, a road, a public building or a power line | A site with overlapping cranes, or adjacent sites with overlap |
| Inspection | Calibration at a fixed point and a check after every change | As with zoning, and a re-check whenever a crane is added or raised |
On urban sites both functions are usually required together, and advanced systems perform both.
The first step is to avoid overlap through the positioning of the cranes, which is also the order set by the guidance in Italy. When overlap is unavoidable, the cranes are placed at different heights, so that the jib of the lower crane passes beneath the lowest parts of the higher crane. The placement and operation of overlapping cranes are regulated in the guidance of the Occupational Safety and Health Administration on the placement and operation of tower cranes working in extreme situations (No. 567226, 2017), which sets, among other things, minimum distances between overlapping cranes. The binding distances must be taken from the current text of the guidance and from the approved placement plan for the site. By way of illustration, the description of an earlier procedure published in the Institute for Safety and Hygiene journal “Safety” (2016) mentioned the following conditions:
Geometric clearances do not solve the problem of the hoist rope. The hoist rope of the higher crane descends along its full height, and the jib of the lower crane may strike it. A lessons-learned document of the Safety Administration describes such a case: the jib of a low crane struck the hoist ropes of a second crane that was moving a concrete bucket, the bucket swung and a worker fell from a height of about 4.5 metres and was seriously injured. This is why, even with a correct height difference, a system and a working procedure are required. Heights change over the course of the project. Every raising of a crane changes its relationship to the other cranes, so the order of raisings is planned in advance so that the order of heights and the differences are kept at every stage. After every raising the system data is updated and the system is tested. On dense sites, flat-top cranes such as the Potain MDT218 or MDT268J12, which have no protruding tower head, tend to be chosen, making it easier to keep a height difference without raising all the cranes.
The British guidance on the use of tower cranes states that electronic anti-collision devices are a secondary aid to planning and communication, and not the first line of defence. In practice this means that before a system is installed, a written crane plan is needed that includes:
The Safety Administration’s lessons-learned document recommends following the procedure for the placement and operation of tower cranes working in overlap, including keeping the distances between the cranes, installing electrical slewing limit switches to prevent collision, marking the cranes, appointing a coordinating signaller and using wireless communication. It also stresses that crane operators and signallers need valid certificates and a briefing on the overlap working procedure. The British guidance recommends a separate radio in each cab on a dedicated frequency, so that a crane operator in the higher crane who sees a low jib approaching their rope can warn their colleague immediately.
A disabled system protects no one. In the French EPICEA accident database, three accidents involving zone-management systems on tower cranes were found between 1978 and 2010, and in all of them the system had been disabled at the time of the accident. The rule on site should be simple: do not disable the system as a working method. When the system is faulty, stop work in the overlap zone or near the prohibited zone and call a technician. The guidance in Britain and France addresses exceptional cases in which a temporary disabling is needed, and requires that it be done only with prior written approval by a defined responsible person, with notice to the whole lifting team, compensating measures and an entry in a log: start time, time of return to operation, who approved it and why. In Britain a flashing white light is also recommended on a crane whose system is disabled. The same principle applies to changing settings. Only a qualified technician changes boundaries or clearances, after approval, with a check that the change was carried out correctly and an update of the zone drawing held by the lifting team.
Sensors can move, and the system can go out of calibration without anyone noticing, and then allow entry into a prohibited zone. So checks are part of the routine:
A jib over a private yard raises concern among neighbours, and sometimes ends up in court. In a case described in the journal “Safety”, a resident sued to have a crane moved whose jib passed over his yard. The solution proposed in an expert opinion was an integrated limit switch: when there is a load on the hook, the trolley does not go beyond the radius that leads to the yard. When the trolley is already beyond that radius, the jib does not slew over the yard. In both cases a safety margin of 2 metres is kept. A jib without a load can pass over the yard, and at the end of the day slew freely with the wind. The legal questions of passing over private property, neighbours’ consent and working over a public road are not uniform, and depend on circumstances and local bodies. In Britain, for example, a licence to work over a road is obtained from the local authority and does not exempt the contractor from coordinating with private property owners. In Israel it is recommended to obtain legal advice and the advice of the safety officer before placement, to check with the local authority which approvals and traffic arrangements are needed for work over a road or pavement, and to inform the neighbours in advance of the restrictions defined in the system.
An overhead power line is a case where zone limiting alone is not enough. The French guidance states explicitly that zone-management systems are not a sufficient preventive measure against an exposed power line, and that they should be used only in addition to other measures, with increased safety distances that take the swing of the load into account. By way of illustration, under the American regulation (OSHA 29 CFR 1926.1408), when part of the crane, the rope or the load could come within less than 20 feet (about 6 metres) of a live line, the line must be de-energised or a minimum distance kept with preventive measures. The distance is set by the voltage, for example 10 feet (about 3 metres) for a line up to 50 kilovolts. The measures include a planning meeting, a marked warning line, a dedicated spotter and a device that automatically limits the range of movement. These are American figures, and should not be copied to Israel. In Israel, the tower crane regulations define an electrical danger zone of 2 metres from an installation or conductor at a voltage of less than 22,000 volts and of 3 metres at a voltage of 22,000 volts and above, and prohibit erecting or operating a crane so that any part of it, including the ropes and the load, is or may enter this zone (regulations 45 to 47). These are minimum distances, and the Israel Electric Corporation may require a greater distance. Before placing a crane near a power line, contact the Israel Electric Corporation to find out the voltage of the line and the possibility of de-energising, relocating or insulating it, and coordinate the planning with the safety officer. A prohibited zone in the system around the line adds a layer of protection, but does not replace this coordination.
RMA installs on building sites advanced anti-collision systems, tested and approved by the Standards Institution of Israel and the Ministry of Communications, for managing overlapping cranes and limiting zones. The engineering and service team examines the overlap on the basis of the site plan, defines the zones with you, and installs, calibrates and tests the system before work in overlap begins. More details are on the anti-collision systems for collision prevention page, and general background on overlapping placement is in the article Overlapping cranes on a building site. If you are planning a site with overlapping cranes, or a crane next to neighbours, a road or a power line, you can start with a needs assessment and initial planning at no cost (an initial review of the site plan and a basic recommendation on model and position; a full placement plan is priced separately), or call the office on 08-9202882.
The basis is the Safety at Work (Tower Cranes) Regulations, 1966, together with the Safety at Work Ordinance. For overlapping cranes, the Safety Administration’s publications refer to the procedure for the placement and operation of tower cranes working in overlap, issued by the Chief Labor Inspector. Crane operators and signallers need certification under the Safety at Work (Crane Operators, Operators of Other Lifting Machines and Signallers) Regulations, 1992. Drafts of amendments to the tower crane regulations were published for public comment in 2022 and 2025, and as of October 2026 we have not found a final text published in the official gazette. Check with the safety officer or with the Ministry of Labor for the current version of the procedure and the regulations. A broader overview is on the tower crane page.
Zone limiting (zoning) works on a single crane and is intended to prevent the load, and sometimes parts of the crane, from entering fixed areas such as a road, a neighbour’s yard or a power line. An anti-collision system links two or more cranes by radio and is intended to prevent contact between them. Advanced systems perform both functions in the same unit.
The accepted safety approach is to prevent the load from passing over the neighbour, and to allow the jib without a load to pass and to slew freely with the wind at the end of the day. The legal question of passing over private property depends on circumstances, so it is recommended to obtain legal advice and the advice of the safety officer before placement.
According to a description of the Chief Labor Inspector’s procedure published in the Institute for Safety and Hygiene journal, at least 2 metres are required between the jib tip of the lower crane and the mast of the higher crane, and at least 2 metres between the high point of the lower crane and the low point of the higher crane. These clearances do not prevent damage to the hoist rope, so a system and a working procedure are also required. Check the current text with the safety officer.
No. The guidance in Britain defines the system as a secondary aid to planning and communication. The coordinating signaller, the radio link between the crane operators and the order of priority in the site procedure remain the foundation, and the system adds a layer of protection.
It is customary to carry out a daily calibration check at a fixed point on site, a full check after every configuration change such as adding a crane or raising, and a periodic check in coordination with the service provider and the certified inspector. Every check is recorded in the crane’s file.
Stop work in the overlap zone or near the prohibited zone and call a technician. Do not disable the system in order to keep working. If a temporary disabling is required in an exceptional case, it is done only with written approval by a defined responsible person, with notice to the whole team, compensating measures and a record.
You usually need to check with the local authority which approvals and traffic arrangements are required for work over a road or pavement. In addition, a prohibited zone for loads over the road is defined in the system. Requirements vary from place to place, so it is worth clarifying them at the planning stage.
The information in this article is general and does not replace engineering, safety or legal advice for a specific site.
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