Safety Data

The handbook states that the typical weighted vibration, a_h = 2.5 m/s² (with an uncertainty, K= 1.5 m/s²).  It says that the declared vibration total value may be used for comparing one tool with another and may also be used in a preliminary assessment of exposure.  It also warns that “the vibration emission value during actual use of the power tool can differ from the declared value depending on the ways in which the tool is used depending on the following examples and other variations on how the tool is used”, namely:-

• How the tool is used, and the materials being cut
•  The tool being in good condition and well maintained
• The tightness of the grip on the handles and if any anti-vibration accessories are used
• The tool is being used as intended by its design and its instructions 

So, I was right that I could have made a purchase decision on vibration, if I’d been able to look in the handbook but I don’t recall if the vibration information was stated on the packaging? I certainly recognise the measurement units as those for acceleration from my O-level physics (that dates me) but how does that help in an assessment of exposure and what is the meaning of typical weighted vibration?  That the actual vibration can be higher than shown is also worrying.

Time to turn to the HSE who publish some very useful guidance on the regulations for hand-arm vibration ^[2]. Regulation 4 from the guidance states that the daily exposure action value (EAV) is 2.5 m/s² A(8) and that the daily exposure limit value (ELV) is 5 m/s² A(8) and that daily exposure shall be ascertained on the basis set out in Schedule 1 Part 1.  The use of A(8) is intended to differentiate between the vibration magnitude of the tool itself and the daily exposure.  It further explains the meaning of A(8), the daily exposure to vibration, using the formula:-

A(8) = ah √ (T/T0), where, 

•   a_h is the measured/stated vibration magnitude
•   T is the duration of exposure to the vibration magnitude a_h; and
•  T₀ is the reference duration of 8 hours

The vibration magnitude a_h is ascertained using the formula 

a_h = √ (a²_hwx + a²_hwy+ a²_hwz), where, 

a_hwx a_hwy and a_hwz are the root-mean-square acceleration magnitude measured in three orthogonal directions x, y and z at the vibrating surface in contact with the hand and frequency-weighted using the weighting W_h.  The definition of the frequency weighting W_h is given in British Standard BS EN ISO 5349-1:2001 ^[3].  The risk to health from vibration is affected by the frequency content of the vibration, measured over a range from about 5-1250 Hz whereby lower frequencies are given greater ‘weight’.

Measurements

Thinking about the data for this particular strimmer, the declared vibration value is already at the EAV and given the uncertainty, the total is approaching the ELV if you used the tool for a full 8 hours.  That’s probably unlikely but given that real-world values can be higher e.g. due to tool wear or be a combination of tool types, it’s advisable to consider taking some measurements.  Appendix 2 to the guidance covers vibration measurement and instrumentation and states that you should take measurements where it is not otherwise possible to adequately assess the exposure and establish whether the EAV is likely to be reached or exceeded or the ELV is likely to be exceeded. 

If you are using a vibration meter, it should meet the correct specification given in British Standard BS EN ISO 8041 ^[4] and specifically have the correct weightings.  Specialist noise & vibration consultant, Essel Acoustics ^[5] has found examples of risk assessments that have been based on instruments intended for machinery applications and hence are wholly unsuitable. Essel also warns against the use of wrist or glove mounted vibration monitors which cannot be used to acquire operator vibration exposure data to the ISO 5349 standard. This is because they do not measure the actual vibration at the point where it is transmitted to the hand.  

It is normal to use an accelerometer to measure vibration which must be rigidly mounted to the handle of the tool at each hand position, and it is preferable to measure all three orthogonal axes x, y & z at the same time.  A vibration meter with a tri-axial accelerometer will do this, and also perform the calculation mentioned earlier, namely a_h = √ (a²_hwx + a²_hwy+ a²_hwz).

How many measurements?

The guidance states that a single measurement for a machine, an operator and a task provides limited information on vibration risk.  This is because vibration magnitudes vary due to factors such as changes in forces, posture and techniques adopted by the operator as well as variation in materials and product.  When making measurements, you should plan to measure several operators, each working across a range of common operating conditions and there is lots more detail in the guidance.

With reports of 6-figure fines for a lack of a competent vibration risk assessment and/or health surveillance regularly featuring in the Safety Press, it’s worth investing time and effort in some measurements but they must be conducted properly.  They must be taken using compliant instrumentation with the sensor correctly fixed in place and while the terminology may seem daunting there is lots of explanatory guidance in the HSE publication, L140 ^[2], perhaps the first £20 you spend.  In the next article we’ll look in more detail at the features to look out for in a compliant instrument.

References

1.       What is HAVS, and why is it important? (casellasolutions.com)

2.       The Control of Vibration at Work Regulations 2005:  Guidance on Regulations.  HSE publication L140, second edition 2019

3.       BS EN ISO 5349-1:2001 Mechanical vibration. Measurement and evaluation of human exposure to hand-transmitted vibration General requirements

4.       BS EN ISO 8041-1:2017 Human response to vibration — Measuring instrumentation — Part 1: General purpose vibration meters

5.       Hand Arm Vibration (HAV) measurements – are you performing them correctly? - Essel Acoustics