Electricity dissipation in a pool after a lighting strike - time?

miss no stars

Hi

We were going over the action plans in work (lifeguard during summer) for rescuing swimmers after a lightning strike (unlikely as that is as pool closes when thunder-storms are forecast). The question came up as to how long after lightning strikes is it ok to get in the water. There's a generalized time of 30 minutes given in some lifeguarding literature but surely it depends on the pool?

The pool in question is chlorinated (bit too much so tbh), has metal entrance/exit ladders and was constructed from in situ concrete. For a "big" strike, does anyone know a way of calculating the time for dissipation of the electricity? (if that makes sense)

30 minutes is a long time to leave someone in there if they could be helped sooner and conversely, I don't want to be getting into a pool that could kill me. :-/

mawk

I cant see the pool acting like a capacitor or boiling and surely those are the only two hazards. my guess for a safe time to enter the pool if about .5 seconds.

though maybe a few minutes is smarter, if one strike hit the pool then its likely the path of least resistance for the storm right then, especially after the first ionized the air between the cloud and the pool. so maybe 5 minutes to reduce the risk of secondary strikes while youre in the pool.

im open to being corrected

enda1

Surely the pool would be one of the safer places to be seeing as it would kinda act like a low resistance path all around you.

Unless your head is very pointy...

But as long as you were floating, the lightning would strike the water I suppose instead of you.

bnt

My understanding is: the concern is not about the pool being "charged" in any way, the concern is about further lightning strikes. For example, this Australian document says the following:

The second part of the 30/30 rule provides the criteria for resumption of play. Here it is recommended that people wait 30 minutes after the last sight of lightning or sound of thunder. This figure is based on the observation that a typical storm moves at about 40 km/h. Thus, waiting 30 minutes allows the thunderstorm to be about 20 km away, minimising the probability of a nearby strike. It is important to emphasise that blue skies and lack of rainfall are not adequate reasons to breach the 30-minute return-to-play rule.

A swimming pool is a target for lightning strikes because it is well grounded. If it wasn't, it wouldn't get hit in the first place. Electrons can flow in and through it quickly enough to form a bolt of lightning, so I think it's safe to say that the water doesn't hold a charge for any significant length of time!

spideog7

It doesn't depend on the pool it depends on the storm. If a pool get's hit by lightning once there's a chance it will get hit again while that storm is overhead. Contrary to the popular saying "lightning never strikes twice" if it finds a path to ground that's easier than anything else then it will continue to follow it until it finds something easier.

But as long as you were floating, the lightning would strike the water I suppose instead of you.

Water is a conductor so if you're touching the water and the lightning strikes the water you will get it too. Similarly if lightning were to strike your mains water supply somehow you could get electrocuted from you're tap, providing it hadn't dissipated along the line.

FoxT

When the pool gets hit by lightning, the electrical charge will dissipate immediately for all practical purposes. So it is safe to go in straight away from that point of view. However, as earlier poster pointed out, there is a risk of a subsequent lightning strike. If there is a casualty in the pool then your choices are:

1 - sit tight & wait for the storm to go away. Outcome: casualty will die.

2 - Jump in & drag him out. No lightning strikes. Outcome: 2 safe people.

3 - Jump in & lightning does strike: Outcome: 2 casualties, perhaps 2 fatalities.

the thing is - doing nothing will result in the casualty expiring 100% of the time (assuming unconscious, face down). In this situation I would jump in myself, & be reasonably confident that I'd be able to get out again

enda1

spideog7 wrote: »

Water is a conductor so if you're touching the water and the lightning strikes the water you will get it too. Similarly if lightning were to strike your mains water supply somehow you could get electrocuted from you're tap, providing it hadn't dissipated along the line.

Surely the fact that the water is a conductor will mean that you (a relative insulator) will not carry much if really any of the current?

bnt

enda1 wrote: »

Surely the fact that the water is a conductor will mean that you (a relative insulator) will not carry much if really any of the current?

That's right - most of the current will bypass you. However, when it comes to lightning, "not much" is still highly fatal. :eek:

spideog7

enda1 wrote: »

Surely the fact that the water is a conductor will mean that you (a relative insulator) will not carry much if really any of the current?

Try jumping into a pool and not getting wet! Also the body is typically made up of 60% water, you're not really much of an insulator. It only takes somewhere in the region of mA to kill a person.

So whilst the different resistance between you and the water makes the water a more attractive option you are still an option. My simplified imagining of this scenario is two resistors in parallel, you being one the water being the other. In that situation the only way you would avoid getting electrocuted is if you were 100% insulated (entirely non-conductive) or the water was 100% conductive (entirely non-resistive) which it's close to but still not quite.

bnt

For comparison, have a look at the situation in cars. You can be quite safe inside a car as it's struck by lightning. (They even did that to one of the Top Gear presenters a few years ago.) It's not just that car is conducting the current to the ground, it does so in a way that routes the current away from the occupants. You're not (usually) grounded, so there's no potential across you and no current will flow through you. In the pool, however, there's definitely scope for a voltage potential across you, and thus a risk of shock. Not much in absolute terms, but still too much for your safety.

miss no stars

Thanks folks.

Wasn't thinking properly when I asked that Of course the pool is grounded :P

Policy has been ammended to - get in, get casualty, GTFO