Understanding Factors of Safety

This is a guide to factors of safety in circus rigging covering the basic concepts with a discussion on different ways to determine the forces applied.

Important Notice

This document is a draft for review and NOT to be put into use. Please send feedback to mark@aerialedge.co.uk. The document was created with input from members of the Circus Rigging and Safety Forum and based on UK and EU regulations

A Factor of Safety

A ‘Factor of Safety’ (FOS) is all about creating a margin for error to protect against malfunction or unexpected excess forces.

For lifting equipment, a manufacturer will repeatedly test their equipment to failure enough times to work out at what load their equipment is likely to break at. They will then add a factor of safety to determine the maximum load that can safely be placed on the equipment when in use.

The factor of safety is written as a ratio and used to determine what forces equipment can safely support. As an easy example, if your equipment needs to be strong enough to lift 1,000kg, and your factor of safety is 4:1, you should make sure that it is strong enough to lift 4,000kg

Let’s look at some other definitions to gain a greater understanding of force and equipment.

Minimum Breaking Load

The Minimum Breaking Load (MBL) is the force a piece of equipment can be exposed to above which it is statistically likely to break. One method to determine the MBL is by testing equipment to destruction multiple times. This allows the manufacturer to collect sufficient data to understand the minimum load it takes to break the equipment 99.7% of the time (using 3-sigma a statistical method to calculate probability).

This testing makes no allowance for the reduction in strength made by terminations such as knots or splices in a rope or wire rope. With forged metal items tests will be conducted with the load applied in the designed axis, the angle along which the equipment was designed to be used, for example on the long axis of a karabiner or an oval ring.

The Minimum Breaking Load (MBL), also known as the Minimum Breaking Strain (MBS), is typically marked in kilonewtons (kN) because it is a measurement of force rather than mass.

10kN ≈ 1000 kgs, 5kN ≈ 500 kgs

The Minimum Breaking Load is the figure that you will see typically see marked on Personal Protective Equipment (PPE) and climbing equipment such as climbing slings, karabiners, pulleys, and swivels.

The Minimum Breaking Load does NOT include a factor of safety and it does NOT mean it is safe to apply forces on the equipment up to this figure. This is where the Working Load Limit comes in.

Working Load Limit

The Working Load Limit (WLL) is used in the lifting industry to show what the maximum load equipment in normal use can be exposed to. This figure is marked on the equipment by the manufacturer and does include a factor of safety. This was originally specified by the EU Machinery Directive.

The Factor of Safety specified for forged (metal) equipment like shackles and chains is 4:1, steel wire rope is 5:1, and ropes and slings made from artificial fibres is 7:1. In the UK this is made law by the Supply of Machinery (Safety) Regulations 2008.

This means a CE or UKCA marked shackle, ring, or eyebolt with a marked WLL of 1 tonne has an MBL of at least 4 tonnes. Any lifting operation will produce more force than just the measured mass of the object being lifted.

The factor of safety is intended to allow for the vagaries of lifting operations, such as wear and tear, shock loads, or aggressive movements. You can therefore place a load of up to 1 tonne on it with confidence because it will not break through overload until a load of at least 4 tonnes is placed on it. Everyone can sleep at night.

Calculating Working Load Limit

The manufacturer calculates the Working Load Limit by dividing the Minimum Breaking Load by a Factor of Safety (FOS). The Working Load Limit must be marked in kilograms (kg) in Europe and the UK.

The Working Load Limit = Minimum Breaking Load / Factor of Safety


In our first example, let’s say we have a forged steel bow shackle with a WLL of 1000kgs. If it is UKCA/CE marked, it will usually have a FOS of 6:1. This means the MBL of the shackle is WLL x 6, i.e., 6000 kgs.

In our second example, we have a round sling with a marked WLL of 1 tonne. Artificial fibre slings must use a FOS of 7:1 therefore we know the MBL of the sling is 7 tonnes when new. WLL of 1000kg x FOS of 7 = MBL of 7000kg.

Equipment designed for lifting

Around the world legal requirements vary. The factor of safety required by law in the EU and UK for lifting equipment put on the market is between 4:1 and 7:1 depending on the material of manufacture.
When lifting equipment that is not designed to lift people is used for lifting people in the UK and the EU the factor of safety normally used for lifting must be doubled.

LOLER 1998 (The UK’s ‘Lifting Operations and Lifting Equipment Regulations’ 1998) Regulation 5 section 157 states “Equipment used for the lifting of people should have a factor of safety relating to its strength of at least twice that required for general lifting operations. This is the arithmetic ratio between the minimum breaking or failure load and the maximum working load marked on the equipment.”

As an example, for a shackle used to lift people, we must calculate the force we will apply to the equipment. This includes the weight and movement of the performer(s) and the dynamic effects associated with the speed of the lift. We then compare this with the MBL on the declaration of conformity. The factor for such equipment is normally 4:1 so we double that to 8:1.
If we know that the act produces a maximum force of 1.2kN (120kgs) at that shackle, its MBL must be at least 8 x 1.2kN = 9.6kN.

The question we need to ask is how do we calculate the maximum force our act will produce?

Acrobatic loads

Acrobatic performers may generate forces much greater than their own body weight; this could exceed a factor of 5 times their body weight. Using a load cell, a device used for measuring force, we have recorded forces of up to eight times the bodyweight of professional artists on certain types of equipment.

Dynamic Loads

Dynamic loads are created by objects in motion. In our case, that is our performer(s). An artist suspended in a harness or performing on circus equipment creates a dynamic load of variable force, which could be called a ‘variable action’.

Static Loads

Static loads are loads where the forces acting on them are constant over time - perhaps better thought of as a ‘permanent action’. In other words, they do not move and are unlikely to be found in acrobatic apparatus.

WLL & Acrobatic forces

One term of note from our above definitions is that of ‘normal use’. What is normal use? In lifting operations, usually lifting inanimate objects. The WLL as shown on most equipment is NOT designed for human loads so when we design rigging solutions the ‘competent person’ who is designing the system will have to apply some additional calculations.

They will need a good understanding of the normal use of the equipment and have good working knowledge about the actual forces that are intended to be applied to the equipment.

Manufacturers’ use cases are unlikely to include acrobatics. When designing rigging for a specific application, we need to look carefully at that equipment and ensure that it is strong enough for how we want to use it.

For each system we design and for each component, we should ask:

  • - What is it lifting?
  • - How is the lifting (essentially vertical travel) carried out (hoist, block, and fall, counterweight, capstan winch, direct effort)?
  • - And crucially at what speed(s)? It is the speed of starting and stopping a lifting or falling motion that creates the extra force as we work against the force of gravity.

We need to understand:

  • - What acrobatic activity is involved - swinging, beats, doubles, drops, etc.
  • - How often it took place and why - school, performance, training, rehearsal, tour
  • - What were the location and premises?
  • - If the force generated has been measured previously and if so, how it was measured. Do we have a load cell calibration certificate?
  • - How was the load cell set up - peak hold, sample rate, etc.
  • - What the equipment is connected to (materials and diameters of other components like hoop/trapeze tags, fibre elements, thimbles in eyes, other connectors/plates/eyebolts)
  • - Was rotation was involved and if bending was a consideration.

This is not an exhaustive list of considerations, and each application will be different. So, plenty of experience and an attitude of curiosity is required.

The MBL of the equipment won’t change but we will need to revise the factor of safety to create our own version of a WLL. This we can call either a Safe Working Load (SWL) or a Design Load to differentiate it from the manufacturer’s markings and to denote it applies to a specific application for which the equipment is being used.

Dynamic Forces and factors of safety

When designing any rigging solution, we need to calculate two figures:

  1. What are the forces we are applying to the equipment?
  2. Is the equipment strong enough?

With dynamic loads, the forces applied to them are changing over time, to determine what equipment to use and how strong our rigging point should be we need to be able to work out what the maximum dynamic force is for our specific application. We can call this the Peak Load. This we are defining as the maximum momentary peak load when the activity is measured.

We can choose to work out the Peak Load in several different ways:

  • - Calculate the applied forces
  • - Measure the forces
  • - Use a ‘rule of thumb’ guide to work out the Dynamic Factor

Calculate the applied forces

There are several ways to work out what forces are placed on a piece of equipment during a performance or in a class. We could look at the biggest drop in an act and use google or some high school physics to work it out. We could include the distance of the drop, the stopping distance, and the bodyweight of the performer.

There are some drawbacks to calculating a load theoretically. You would need to be sure you were using the right formula, that your calculations were correct, and you would also have to understand acrobatic forces well enough to ensure you were calculating the greatest forces in an act. There are many possible ways to calculate the forces involved and there is no clarity on which is the best approach for aerial acrobatics. I would therefore recommend we measure the forces involved.

Measure the forces

The surest way to measure a load in circus is to use a device called a load cell, a small but worthwhile investment to understand the forces generated in an act.

Load cells will give a reading based on the amount of force applied to them and can record Peak Loads and chart the varying forces in a graph with force shown on one axis of the graph and time shown on the other.

It is worth considering that some factors may cause an increase in peak load that may not be captured when measuring the activity. An example would be a higher level of adrenalin in the performer during a show causing more powerful movements in each technique. Higher Peak Loads may also occur also if a ‘trick’ goes wrong causing the performer to ‘save’ themselves leading to more aggressive movements.

If we were to collect load cell measurements from multiple tests and put them into a database, we could start to see what forces are generated over a larger pool of artists and students. This would help us more accurately determine rules of thumb for different use cases. We have been doing that for some time within this group and have a database of test data. We would however love to have more people add to the database to cover different arts and equipment as well as more people attempting to reproduce the same results.

Use a Rule of thumb guide to Dynamic Factors

The Dynamic Factor is a way to estimate the force (Peak Load) applied by the performer.

In the absence of a known load, we can use a rough rule of thumb by applying a ‘dynamic factor’ where we could estimate the likely dynamic factor for the intended activity and apply that to the bodyweight of the performer. This would require a lot of experience in the forces generated by different acrobatic techniques on different acrobatic equipment. It would also require access to sufficient data to enable us to create the guidelines.

The mass of the performer multiplied by the dynamic factor would now be considered the Design Load in our rigging design. We would then apply a factor of safety to this figure.

The following are examples and should not be considered recommendations.

The terms ‘static’ and ‘dynamic’ in this section relate to commonly used descriptions in aerial acrobatics where ‘static’ techniques refer to where the equipment itself is hung and relatively static and the dynamic actions are provided by the performer. ‘Dynamic’ techniques usually refer to disciplines like swinging trapeze or flying trapeze. I’ve added the word ‘aerial’ after for clarity.

Example 1 – Low impact static aerial techniques

For low-impact static aerial exercises, we could use a factor of 3:1. Where we multiply the weight of the performer by three to calculate our dynamic factor. This could include static aerial exercises for fitness in a studio or for conditioning or beginner movements such as hanging, sitting, or standing on the aerial equipment 
(e.g., silks, hammocks, static trapeze, hoop)

Example 2 – Medium Impact static aerial techniques

Acrobatic routines for fitness or performance increase the Peak Load for example, ‘beating’, which is repeatedly swinging under a piece of equipment, generates significantly more dynamic force. We could use a dynamic factor of 5:1 for techniques like this. Any movement that includes ‘drops’, ‘beats’, or movements that are more aggressive would fit into this category.

Example 3 – High Impact ‘static aerial’ techniques and all dynamic techniques

Professional performer bodies have been conditioned over time to take loads which would lead to injury in those who are just beginning in aerial arts and can therefore perform techniques that generate greater forces. In addition, certain static aerial equipment with low elasticity in their design and types that can be attached to the performer, such as aerial straps, can help the performer generate much higher peak loads.

Dynamic techniques such as flying trapeze, swinging trapeze, and cloud swing would fall into this category too. This also includes aerial acts that are ‘flown’ or ‘counterweighted’. e.g., straps and similar where the performer is lifted with lifting equipment as part of the performance. In this application, the dynamic effects associated with the speed of lift will need to be included in our design.

For high-impact aerial applications, we could use a dynamic factor of 10:1.

Example 4 – Performer flying

Performer flying in theatre, circus, corporate or live events as part of a performance, rehearsal, or training create large and repeated dynamic forces of significance. For performer flying, we could also use a dynamic factor of 10.

Dynamic forces & Factor of Safety

In the previous section, we have covered three possible ways to calculate or measure the Peak Load in an act. Once we know the Peak Load generated, we need to then calculate the factors of safety between our dynamic load and the Minimum Breaking Load (MBL) of all the equipment we are using.

As in our previous example, for a shackle when used to lift an acrobatic load, we must compare the force we will apply to the equipment with the MBL on the certificate or declaration of conformity and ensure that it is at least 8 times that load.

So if our load cell tells us the act produces a maximum force of 1.2kN (120kgs) at that shackle, its MBL must be at least 8 x 1.2kN = 9.6kN.

CE or UKCA marked karabiners will have an MBL of 22kN or greater, so would be appropriate for lifting people if they are in good condition and rigged appropriately with respect to the application and the environment in which they are used.
Equally, it could be said that when the force exceeds 275 kgs, a typical karabiner will not provide 8:1

Does LOLER apply to all circus rigging?

Only if people are actually being lifted. That is, acts where a performer is being raised and lowered in a harness or on their equipment. It could be argued this is outside the scope of LOLER but using the same principles would be good practice.

In many examples of circus rigging, the equipment is ‘static’ in that the dynamic loads are applied by the performer. Climbing a rope, jumping on a trapeze, or hanging on a hoop cannot be considered ‘lifting’. Therefore, complying with LOLER is not a legal requirement, but PUWER (The Provision and Use of Work Equipment Regulations 1998) will cover its use.

However, taking the same equipment and attaching it to a lifting device/machine/assembly makes the whole thing lifting equipment and then becomes subject to LOLER. Counterweight act rigging uses manual effort and all equipment used can therefore be regarded as lifting equipment and therefore LOLER probably applies.

All equipment in a work scenario would be covered by PUWER regulations. PUWER could apply to a silk hanging in a studio as much as a mat or trampoline so this could be considered a minimum standard for suspended equipment as well as for mats and other work equipment.

Lifting is a more complex scenario when calculating forces over statically hung equipment and the forces applied may well be greater, however, LOLER does cover topics such as forces, factors of safety, and inspection that we would have to reproduce in our risk assessments and method statements to comply with working at height legislation (The Work at Height Regulations 2005).

Applying lifting regulations to suspended equipment may be the easiest way to keep people safe and lower the cost of operations. After all you may end up using the same equipment for counterweighting someone in a show as when training someone in your school. In which case, the items should be marked to show which can be used for lifting. LOLER 1998, Reg 7 (e) and PUWER Reg 23.

Keeping separate equipment for each type of use as well as the extra cost of administration and training may cost more than not applying LOLER to all aerial acrobatic equipment.

This article first appeared on www.circusrigging.info