30th March, 2021

Understanding Your HAVS Trigger Times

HAVS trigger times are how long your hands and arms are exposed to vibration levels. Trigger times can often get overestimated because they are not always fully understood. In this post, we look at what trigger times are and how to measure them when assessing vibration exposure.

Understanding Your HAVS Trigger Times header image

Do you use vibrating tools and equipment? Like a drill, a saw, or even a lawnmower. If you do, you need to be aware of the risks of vibration, the legal limits and carry out HAVS assessments. Because vibration exposure can lead to severe and irreversible health problems.

Trigger times will come up in almost every HAVS assessment you do. It will be a consideration in every vibration exposure you measure. If you don't know what a trigger time is or how to measure it, don't worry, read on for a detailed explanation.

Measuring vibration exposure is part of your HAVS assessment. You can't assess the risk from vibration without first knowing what the vibration level is. Most people jump straight into the tools handbook and look at the vibration levels. And you could assume that a tool with a vibration output of 12m/s² is worse than one with a vibration output of 5m/s². Even if you don't know what m/s² means, you know that the number 12 is bigger than the number 5, so that's where the risk is, right?

Wrong. Well, maybe right. But we don't have enough information yet! To understand why you need more information, you need to understand trigger times.

Hand-arm vibration syndrome (HAVS) can affect anyone exposed to vibration, but the longer your trigger times, the bigger the risk. Let's start by looking at what HAVS trigger times are.

What are HAVS trigger times?

HAVS trigger times are a measure of how long you are exposed to vibration. This is not just how long you are holding a tool or piece of equipment, but how long you are in contact with it while it is on, and while it is vibrating.

chainsaw trigger time

Let's say you are using a drill for an hour.

Are you drilling for every minute of that hour? Chances are, some of that hour gets spent measuring up and marking the positions for drilling. And once you have drilled, some time is spent fixing items in place. You might have the drill out for an hour, but your trigger times could be less than 15 minutes. Measure twice, and drill once!

Why are trigger times so important?

The law requires you to make sure that risks from vibration are minimised and controlled. So you can get vibration levels under control and reduce the risk of HAVS, you first need to understand your exposure.

To measure vibration exposure, you need to know your trigger times. Why? Because exposure limits are time-weighted. That means that exposure limits are based on the length of time you are exposed to vibration. The longer you use a vibrating tool, the bigger the risk.

Let's take our example from the start of this post. We have a 12m/s² tool and a 5m/s² tool. When is the 5m/s² worse than the 12m/s² tool? When its trigger time is long enough to make it worse.

If you use both tools for 30 minutes each, yes, the 12m/s² is worse for vibration exposure. But what about if you use the 12m/s² for only 10 minutes (because it's more powerful and gets the job done faster) and the 5m/s² tool for 120 minutes (because it's lower risk, and you needed to!).

Now our 5m/s² tool is worse. A lot worse. It accounts for 68% of your vibration exposure compared to 32% for the 12m/s² tool.

HAVS trigger time chart

We go more into detail on calculating vibration exposure in our posts about measuring exposure action values and exposure limit values.

If you want to cheat, you can use our free HAVS calculator.

But, before you do, you need to know your trigger times!

How do you measure trigger times?

Most people estimate trigger times, this is the quickest way, but this can be inaccurate.

If you estimate based on the amount of time you had the tool out of the box, you're going to be overestimating your trigger time.

If you estimate based on how long you think you were using the tool, you're probably going to underestimate it. You know what people say - time flies when you are having fun!

The easiest way to accurately measure trigger times is to monitor your work. There are even electronic monitoring systems you can attach to tools and users to do this for you. But if you're not ready to go hi-tech, you could keep a register of your exposure. Do this by making a note of how long you use each tool in a logbook or form.

You can even use our HAVS calculator to do that. Each time you use the tool, add it to the calculator. For example, 'Drill task 1' and 'Drill task 2' - you get the idea.


Find out how to measure and calculate vibration exposure under the legal limit, and use the free vibration toolbox talk to raise awareness.

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This article was written by Emma at HASpod. Emma has over 10 years experience in health and safety and BSc (Hons) Construction Management. She is NEBOSH qualified and Tech IOSH.

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