3.2 BS EN 280-1 & ISO 16368 Design Groups (A and B)

BS EN 280 & ISO 16368 Kinematic Design Groups

Quick Answer: Under the MEWP design standard BS EN 280-1:2022 (and its international equivalent ISO 16368), MEWPs are divided into two groups by where the centre of the platform area can go relative to the machine's tipping lines: Group A, where its vertical projection is always inside the tipping lines (scissor lifts, vertical masts), and Group B, where it may be outside them (articulating and telescopic booms).

The engineering design of MEWPs must guarantee structural integrity and resistance to overturning under rigorous combinations of payload, environmental forces, dynamic motions, and ground inclinations. Understanding how design kinematics dictate machine behaviour is essential for every professional MEWP operator and manager.

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The Engineering Framework: BS EN 280 & ISO 16368

BS EN 280-1:2022 (Mobile elevating work platforms – Part 1: Design calculations – Stability criteria – Construction – Safety – Examinations and tests) is the design standard manufacturers use to show conformity with the UK Supply of Machinery (Safety) Regulations 2008 and the EU Machinery Directive 2006/42/EC. It replaced EN 280:2013+A1:2015, whose presumption of conformity ended in February 2025, and works alongside BS EN 280-2:2022 for load-handling devices fitted to MEWPs. ISO 16368 is the international counterpart.

The standard sets the design values that manufacturers must meet before affixing a UKCA or CE marking:

• Rated load: calculated as 80 kg for each person the platform is rated for, plus at least 40 kg in total for tools and materials. A one-person platform is therefore rated at no less than 120 kg, and a two-person platform at no less than 200 kg.
• Wind loading: outdoor machines are normally designed for a wind pressure of $100\text{ N/m}^2$, corresponding to 12.5 m/s (28 mph, Beaufort Force 6), acting together with rated load. Indoor-only machines are rated at 0 m/s. Manufacturers may design for higher wind speeds, so the rating on the machine is the limit.
• Manual force: the design assumes a horizontal manual force from occupants of 200 N for a platform rated for one person and 400 N for a platform rated for more than one person.
• Dynamic Factors: Calculations must account for acceleration and deceleration forces generated by hydraulic valves, slewing drives, and emergency braking systems.
• Maximum Permissible Chassis Inclination: Machines are designed to operate safely on ground slopes up to a specified maximum (typically 1.5° to 3° for slab machines and 3° to 5° for rough-terrain units).

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Stability Physics & Overturning Moment Mathematics

Machine stability depends upon the rotational equilibrium of moments acting about a critical fulcrum called the tipping line (or tipping edge).

Overturning Geometry & Moments:

                 [Platform Load (Wp)]
                          |
                          v
                      |<- d_p ->|
-----------------------+---------+---------------------
[Counterweight (Wc)]   | Tipping |   Ground Contact
        |              |  Line   |
        v              | (Fulcrum)
    |<-    d_c    ->|  |

Stabilising Moment: Ms = Wc * d_c
Overturning Moment: Mo = Wp * d_p + Wind + Side Force
Stability Condition: Ms >= gamma * Mo

1. Tipping Lines Explained

The tipping line is the horizontal axis about which a MEWP would rotate if it were to overturn. Geometrically, tipping lines connect the outermost ground contact centres:

• On wheeled scissor lifts: the lines connecting the centres of the front and rear tyre contact patches.
• On tracked spider lifts: the centrelines of the crawler tracks or outrigger feet.
• On outrigger-supported machines: the line connecting adjacent outrigger footpad pivot pins.

2. The Governing Equilibrium Equation

For a MEWP to remain stable in any operational configuration, the total stabilising moment ($M_S$) about any tipping line must exceed the total overturning moment ($M_O$) with the margin the design standard requires ($\gamma$):

$\sum M_{\text{stabilising}} \ge \gamma \sum M_{\text{overturning}}$

Where:

• $M_S$ (Stabilising Moment): Generated by the self-weight of the chassis, engine, hydraulic assemblies, and dedicated counterweights located on the safe side of the tipping line ($M_S = W_{\text{chassis}} \cdot d_{\text{stabilising}}$).
• $M_O$ (Overturning Moment): Generated by the platform weight, personnel, tools, horizontal wind loads, occupant side forces, and dynamic stopping forces acting on the opposite side of the tipping line ($M_O = W_{\text{load}} \cdot d_{\text{overturning}} + F_{\text{wind}} \cdot h + F_{\text{manual}} \cdot h$).
• $\gamma$ (Design margin): The load factors and test overloads specified by the standard rather than one fixed number; for example, power-operated MEWPs are overload-tested at 125% of rated load.

3. Structural Deflection & Base Angle Amplification

Steel and aluminium boom structures are elastic. Under full platform loading and maximum horizontal reach, the boom deflects downward and outward. This structural deflection increases the moment arm ($d_{\text{overturning}}$), compounding the overturning moment.

Furthermore, if the chassis rests on an unlevel surface with angle $\theta$, the horizontal displacement ($d$) of the platform centre of gravity increases drastically with working height ($L$):

$d = L \cdot \sin(\theta)$

On a 30-metre boom, a seemingly minor chassis slope of just 3° displaces the platform head horizontally by:

$d = 30 \cdot \sin(3^\circ) = 30 \cdot 0.0523 = 1.57\text{ metres}$

This 1.57-metre outward shift dramatically magnifies the overturning moment, pushing the machine toward catastrophic destabilisation if operated outside manufacturer slope limits.

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Group A: Platform Centre Always Inside the Tipping Lines

Under BS EN 280-1 and ISO 16368, Group A MEWPs are those where the vertical projection of the centre of the platform area, in all platform configurations at the maximum chassis inclination specified by the manufacturer, is always inside the tipping lines.

Group A Kinematic Profile (Scissor Lift / Vertical Mast):

             [   Platform   ]  <--- Centre of Gravity (CoG)
             [     Load     ]
                 |   |  |
                 |   |  v (Vertical projection)
                 |   |  |
             ====X===X===X====
             |               |
             |    Chassis    |
           [Wheel]       [Wheel]
             ^               ^
             |               |
        Tipping Line    Tipping Line
             |<-- SAFE ZONE ->|
         (CoG projection ALWAYS inside)

Machine Characteristics

• Elevation Mechanism: Pure vertical pantograph scissor stacks, vertical telescoping masts, or push-around vertical (PAV) columns.
• Representative Equipment: Category 3a self-propelled scissor lifts, Category 1a static vertical lifts, low-level push-around verticals.
• Horizontal Outreach: Strictly limited to small roll-out deck extensions (typically 0.9 m to 1.5 m), which are engineered so the centre of mass remains firmly within the chassis footprint.

Stability Vulnerabilities of Group A

Because Group A machines do not extend booms horizontally, their centre of gravity stays within the tipping polygon during normal elevation. However, they are highly sensitive to:

• Base Inclination: Driving or elevating on slopes exceeding 1.5° to 2°.
• Unequal Ground Settlement: A wheel dropping into a pothole, grating, or soft soil patch.
• Excessive Side Forces: Occupants leaning outward or pulling forcefully on building facade components (exceeding the 200 N/400 N threshold).
• Sail Area Wind Loading: Attaching banners, plywood sheets, or cladding panels to the guardrails, which multiplies the wind overturning moment.

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Group B: Platform Centre May Be Outside the Tipping Lines

Under BS EN 280-1 and ISO 16368, Group B covers all other MEWPs: the vertical projection of the centre of the platform area may be outside the tipping lines, depending on boom position.

Group B Kinematic Profile (Articulating / Telescopic Boom):

                                     [ Platform Load ]
                                             |
                                             v (CoG Projection)
                                             |
[Counterweight]                              |
      |                                      |
      v                                      |
   [Chassis]-------------------|             |
   [Wheel]                  [Wheel]          |
      ^                        ^             |
      |                        |             v
 Tipping Line             Tipping Line   (OUTSIDE TIPPING LINE)

Machine Characteristics

• Elevation Mechanism: Telescopic booms, articulated z-booms, fly-jibs, and rotating turrets capable of 360-degree slew.
• Representative Equipment: Category 3b mobile boom lifts (cherry pickers), Category 1b vehicle-mounted platforms, crawler spider lifts, trailer-mounted booms.
• Cantilever Dynamics: The platform can extend 10, 20, or even 40+ metres horizontally away from the chassis centreline.

Equilibrium and Load Moment Control

Because the platform load acts outside the chassis footprint, Group B machines rely entirely on:

• Heavy Counterweights: Positioned on the superstructure opposite the boom assembly.
• Wide Outrigger Footprints: Deployable outrigger beams that push the tipping lines far outward.
• Electronic Load Moment Monitoring: Load cells on the basket and boom angle/length sensors that continuously compute the live overturning moment. If the platform approaches the permissible stability boundary, the control system locks out all "aggravating" functions (boom extend, boom lower) while permitting only "safe" functions (boom retract, boom raise).

Technical Comparison: Group A vs. Group B