27th April, 2018

Understanding Your HAVS Trigger Times

HAVS trigger times are how long your hands and arms are actually exposed to vibration. Trigger times can often be overestimated because they are not 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? If you do, you need to be aware of the risks, the legal limits and carry out HAVS assessments.

Trigger times will come up in almost every HAVS assessment you do, and every vibration exposure you measure.

If you don't know what trigger time is, or how to measure it, don't worry, all will be explained.

An important part of HAVS assessments is measuring vibration exposure. Most people jump straight in the tool handbook and look at the vibration levels.

Of course, 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 12 is bigger than 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 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 vibrating.

chainsaw trigger time
Clocking up some trigger time!

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

Are you actually drilling for that full hour? Chances are, some of that hour is spent measuring up the positions, and fixing items in place.

You might have the drill out for an hour, but your trigger times could be less than 20 minutes.

Why are trigger times important?

The law requires you to make sure that risks from vibration are controlled, and in order to control vibration 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 for, 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 just so shaky) and the 5m/s² tool for 120 minutes (because it's lower risk, and you needed it!).

Now our 5m/s² tool is 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 fairly inaccurate.

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

If you estimate based on how long you think you were using the tool, your probably going to underestimate (you know what they say, time flies when you are having fun!).

The easiest way to accurately measure trigger times is to monitor your work.

You could keep a register of your exposure, by making a note of how long you use each tool.

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

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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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