Zoomlion ZT23J Uncontrolled Descent Traced to Cylinder Replacement Error
30 Second Takeaway
A worker suffered minor injuries when a Zoomlion ZT23J boom lift experienced an uncontrolled descent at a Winvic site in the UK. The 25.1-metre working-height lift was being used to install cable tray beneath roof beams when its extended boom dropped in stages and the platform came to rest on a Dingli scissor lift. Winvic’s follow-up investigation attributed the incident to a transportation seal left on a replacement primary boom cylinder, creating a weak point in the pressurised hydraulic system. The worker was checked at hospital and given the all clear. Winvic later rescinded its temporary suspension of Zoomlion boom lifts on its sites.
Incident Overview
Winvic construction site, United Kingdom
A worker was installing cable tray containment beneath roof beams or purlins from a Zoomlion ZT23J telescopic boom lift supplied from the Hire Safe Solutions fleet. A large Dingli scissor lift was being used alongside it.
The boom lift reportedly descended in two to three metre increments until its platform came to rest on the scissor lift’s chassis. Winvic’s follow-up investigation identified a maintenance-related cause involving a replacement primary boom cylinder. The lift’s cylinder had been replaced before the machine arrived at the Winvic site.
What Happened
Based on information available at the time of publication:
A worker was installing cable tray beneath roof beams from a Zoomlion ZT23J telescopic boom lift.
The boom was almost fully extended when it began to descend unexpectedly.
It dropped in stages of approximately two to three metres.
The platform came to rest on the chassis of a Dingli scissor lift, which tipped rearwards.
The worker suffered bruising and other minor injuries.
He was taken to hospital for a check-up and later given the all clear.
Winvic’s investigation found that a transportation seal had been left attached to a replacement primary boom cylinder.
Winvic said the retained seal created a weak point in the pressurised hydraulic system.
What Is Known
Equipment involved: Zoomlion ZT23J telescopic boom lift
Reported working height: 25.1 metres
Lift provider: Hire Safe Solutions
Other equipment involved: Dingli scissor lift
Work scope: Installing cable tray containment beneath roof beams or purlins
Incident type: Uncontrolled boom and platform descent
Injuries: Minor bruising; worker checked at hospital and given the all clear
Investigation finding: Transportation seal left attached to a replacement primary boom cylinder
Maintenance timing: Cylinder replacement took place before the lift arrived at the Winvic site
Winvic response: Temporary suspension of Zoomlion boom lifts on its sites was later rescinded
Early Observations
Winvic’s follow-up notice attributed the incident to the transportation seal remaining on the replacement cylinder after installation. The notice said the seal was intended to protect the component during storage and transport but should have been removed as specified.
Winvic stated that the work had been carried out by an approved, competent person, but the seal-removal instruction was not followed. The company described the event as a rare, isolated incident. It also said the machine had undergone inspections before arriving at the site, but those checks did not identify the issue.
Winvic initially suspended the use of Zoomlion boom lifts on its sites while the incident was investigated. It later rescinded the suspension after issuing its findings.
Why This Incident Was Possible
The investigation identified a maintenance-related installation error involving a replacement hydraulic cylinder. The retained transportation seal affected the integrity of the pressurised connection and created a weak point in the system.
The incident demonstrates that inspections before use may not identify an issue introduced during component replacement. Maintenance work and the checks that follow it are essential parts of returning equipment safely to service.
Industry Reminder
Component replacement on a MEWP is safety-critical work. Parts must be installed and configured according to the manufacturer’s instructions, including the removal of transportation and protective materials where specified.
After repairs, the machine should undergo structured post-maintenance verification before it returns to service. IPAF issued a member alert following this incident, emphasizing the need to verify replacement components and carry out post-repair checks.
Editorial Note
Crane Hub Global reports on crane, access-equipment and lifting-related incidents to support industry awareness and learning.
This article is based on information available at the time of publication and includes findings reported by Winvic following its investigation.
How Post-Maintenance MEWP Failures Can Be Prevented
Follow component installation instructions
Complete repairs and replacements in accordance with the manufacturer’s specifications, including instructions for seals, protective devices and hydraulic connections.
Verify work after repairs
Use a documented post-maintenance process to confirm that components are correctly installed, configured and functioning before the machine returns to service.
Treat non-routine repairs as safety-critical
Ensure technicians have the appropriate competence, current instructions and time to complete and check uncommon maintenance tasks.
Keep clear maintenance records
Record replaced components, work performed, inspection results and verification steps so that the machine’s service history is clear.
Communicate equipment restrictions accurately
When an incident is under investigation, share the known facts and status of any equipment suspension, then issue a timely update when findings are confirmed.
