Cave Diving Safety: The Importance of Backup Lights

Underwater flooded caves are one of the few places where we can experience complete and utter darkness, if we don’t have lights with us. One of the golden rules of cave diving is that divers always need to carry three lights - one primary and two backups. These lights are used to light up the darkness of the cave environment so that we can see it and provide a means of communication with the team through signals and passive light communication. Taking three lights is to ensure that if one of the lights fails there is a backup with sufficient battery life to get you out of the cave while being able to see the line. Diving in a team means that even if one diver was to lose all their lights, there should be enough lights within the team to share and get out. So let’s look at some scenarios where total light failure is possible.

Sheck Exley’s Blueprint for Survival on lights

Exley wrote his seminal work on accident analysis in 1979, when light technology was totally different from today’s. He used past cave diving accidents and analysed the contributing factors and found lights to be a major factor. At the time of writing he estimated that 1 in 50 dives would result in a light failure. Battery technology was also very different — Some divers I know had custom primary lights built in the 1990s by DiveRite which used 12 volt motorcycle batteries to power them. These super lights had a burn time of 80/90 minutes, which was considered excellent for the time. I would argue that this figure today is far smaller, lights are purpose manufactured for diving applications. Given the high quality of today’s primary lights - and their long burn times - failure in my experience has happened due to human error.

Do light failures happen?

Light failures do happen, but this should be seen more as minor inconveniences and not emergencies, so why do lights fail? There have been occasions where I have gone into a dive with a light that was not fully charged, and as a result the light ran out of battery during the dive. Other failures I have experienced were of the LED burning out and becoming very dim, hardly providing any light at all and issues with failures in the cables. Canister light cables are definitely a weak point, with potential to damage the cable at the battery connection, light head connection, or along the cable. Typically this is caused by rough use - holding the light by the cable rather than the heavier battery and letting the cable take the weight of the battery. Or potentially when a primary light is clipped off and not in use, and the cable is not ‘cleaned up’ and stowed away, the loop of cable could snag on the cave. Another possibility is for a light to flood, either due to poor sealing of the light during assembly or due to damage. Depending on the variety of light, some light switches can fail mechanically, rendering the light unusable.

Team diving and light failures

Imagine you are diving along a beautiful cave passage, your light shining on stalactites and stalagmites, your teammate a body length in front of you. Suddenly your light cuts out, you stop swimming and look down to inspect the light head and as you look up, realise your buddy has continued swimming. Without a light to signal them, you begin to deploy your backup light. By this point, the team should have realised there was an issue and turned to offer support to the diver with the light issue. Light awareness of our team members' lights is a critical part of team diving and makes cave diving safer. Once a light fails within a team, then the team should call the dive and make their way out of the cave. But, Exley points out an option is to carry a fourth light or back up primary, so if a light does fail during a dive, the team still has three lights per diver and can continue. This is very possible today, especially as many of the backup lights on the market now boast 1000 plus lumens on full power and hand held primary lights are affordable.

Backup light considerations

There are a few things we need to consider when it comes to backup lights - power, duration, how they activate and where we store them. A quick internet search of common scuba diving light brands shows that many backup lights on the market boast over 1000 lumens of light power, with options to run the lights at different power levels. I want a backup light to still have decent power, because if I have to use it to exit out of a cave, I want to be able to see and marvel at the cave on the way out. Duration is important because we need a light that has the battery power to get us home from the furthest point of the dive. Generally, people are not doing five hour dives, so a two hour burn time on a backup light should be sufficient for most divers. Most backup lights turn on either through a button, or a twist mechanism.

I like to dive with one of each style - the twist lights if not verified on the surface can turn on as the water pressure increases, and if not caught by a buddy can result in a drained battery when it is needed! (Verifying that ALL lights are functional is a standard part of every pre-dive check). A button style can be quicker to activate or easier if stored on a helmet. Which brings us to consider where we store backup lights. For diving in backmount configurations, (open circuit or closed circuit rebreather) backup lights are attached to chest d-rings and stored on the shoulder straps. For sidemount divers there are a few more options, I personally prefer to keep my backups in my pouch along with my other ‘emergency’ gear. Most pouches have a ‘window’ or mesh to allow teammates to see if a light has accidently turned on while diving. Other places to store the lights would be pockets or mounted to a helmet. In sidemount, I don’t like attaching lights to the shoulder straps of the harness, it just adds clutter.