Dropping hairdrier into the bathtub

[ekmdgrf]

While eating dinner the other night I was watching an episode from series 2 of MythBusters, where they put to test the theory that you will be electrocuted if you should have the misfortune to do such a thing. Now, we all know that MythBusters isn't real science:

http://xkcd.com/397/

So I'd thought I'd put the question out there. What do people think would happen to the person in the bathtub? Let's assume 230V 50Hz AC hairdryer. Let's also assume the absense of RCD protection. Let's consider both the case of a cast iron bath with copper piping (properly bonded) and also a fibreglass bath with plastic piping. Let's also assume the bath is filled with 40C tap water and a drop of radox muscel soak. To preempt BAS, let's also assume that the hair drier being used in zone 0 is not IP67 compliant

 

[Maxatoria]

For rigor we need to know the make/model of dryer, height of release above the water, angle of release, where is the socket for the dryer to be plugged into, is it switched on and probably 100 other things

 

[PrenticeBoyofDerry]

is the hair-dryer class I or class II and what ph rating is the water?

 

[JohnW2]

So I'd thought I'd put the question out there. What do people think would happen to the person in the bathtub? Let's assume 230V 50Hz AC hairdryer. Let's also assume the absense of RCD protection. Let's consider both the case of a cast iron bath with copper piping (properly bonded) and also a fibreglass bath with plastic piping. Let's also assume the bath is filled with 40C tap water and a drop of radox muscel soak. To preempt BAS, let's also assume that the hair drier being used in zone 0 is not IP67 compliant

The second scenario is easy - with a non-metallic bath and plastic piping (and nothing else earthed (e.g. an earthed metal shower hose} in the equation), 'probably nothing' (or virtually nothing) would happen to the person unless they touched something else which provided a path to earth.

With the earthed metal bath, there is a full spectrum of possibilities, varying from 'nothing' (or virtually nothing) to 'death', depending on a whole range of factors which affect how much (if any) of the current flows through the person and what path the current takes through the person.

Kind Regards, John

 

[ericmark]

With earthed bath.

Radox is a large factor here. It seems with boats where there is a problem in fresh water swimmers can be electrocuted if they get close. But in salt water they need to touch the boat.

Consider both line and neutral would be connected to the water and with Radox in the bath water likely the reaction would all be with in the casing of the hair drier.

So it could be lethal but it also could do nothing.

With plastic bath and pipes likely nothing would happen.

 

[bernardgreen]

It is frequently stated that pure water is non conductive. So the amount of dissolved matter is very important.

Experiments with a dead, defrosted turkey in a pool of water as the target for a defibrilator showed that voltage gradient in the water close to the discharge of the defib were low enough to be of no significant hazard to people standing in the water when the paddles were on the turkey.

http://www.ncbi.nlm.nih.gov/pubmed/12879378

In theory a hair dryer in a bath with live and neutral in contact with the water in or ajacent to the drier would not create a gradient high enough to kill. do NOT test this theory with a person in the bath.

But in real life too many variables are involved for any theory to give accurate results. Any path for current to leave the bath would result in some current through the water and this might create a voltage gradient high enough to shock a person in the bath.

Ironically if the bath was not earthed then there would be no current through the water other than inside the hair dryer. No current so no voltage gradient so no shock to persons in the water. No earth leakage so the RCD would NOT trip, the average potential of the bath water would be 115 volts ( mid point live and neutral ) and the person would be safe. But when they stepped out of the bath they may have one foot in the bath and touch an item with a connection to earth. They would then receive a shock, possibly fatal, but at least the current through the body to earth would trip the RCD.

This failure of the RCD to trip while the bath water is at 115 volts above ground is a sensible reason to reconsider whether metal baths should be earthed. Risk assessment needed.

 

[dhutch]

One thing I can add, is that if you drop a drilling into the canal along side a steel boat. Hull earthed, all RCD protected, drill keeps running as you pull it out by the flex and unplug it.


Daniel

 

[bernardgreen]

It would be safer to unplug it before pulling it out of water. Touching the wet part of the lead might be hazardous when the drill comes out of the water.

 

[securespark]

the average potential of the bath water would be 115 volts ( mid point live and neutral ) and the person would be safe. But when they stepped out of the bath they may have one foot in the bath and touch an item with a connection to earth. They would then receive a shock, possibly fatal, but at least the current through the body to earth would trip the RCD.

So the pd between the item with a connection to earth and the person would be 115V ish.

What would the current be, compared to the same scenario with a pd of 230V?

 

[JohnD]

It seems to me that the L & N terminals inside the drier would only be about 20mm apart. So if the water is at all conductive, almost all the current will take that shortest path, and it seems improbable that the route through water, skin, body, skin, tap will be low enough resistance for any discernable current to affect the person.

If it was an old-fashioned electric fire, with the termainals about 500mm apart at each end of the rod, and quite a lot of resistance between them, it might be different.

 

[JohnW2]

Experiments with a dead, defrosted turkey in a pool of water as the target for a defibrilator showed that voltage gradient in the water close to the discharge of the defib were low enough to be of no significant hazard to people standing in the water when the paddles were on the turkey. http://www.ncbi.nlm.nih.gov/pubmed/12879378[/QUOTE]
Indeed, and that's what one would expect. Given that the output of a defibrillator is floating, this is essentially equivalent to the situation with a hairdryer in an unearthed bath - in fact, even 'better', since simultaneous contact of the person with earth would not create a hazard.

But in real life too many variables are involved for any theory to give accurate results. Any path for current to leave the bath would result in some current through the water and this might create a voltage gradient high enough to shock a person in the bath.

Indeed.

Ironically if the bath was not earthed then there would be no current through the water other than inside the hair dryer.

That's obviously not literally true - there will be some current fields in the surrounding water. However, as with the turkey, their magnitude is very unlikely to be high enough to pose any risk.

This failure of the RCD to trip while the bath water is at 115 volts above ground is a sensible reason to reconsider whether metal baths should be earthed. Risk assessment needed.

We've been there many times before and, yes, consideration and risk assessment are necessary. There may be specific other issues in a particular case, but the basic risk assessment essentially revolves about the judgment of which is more likely - electricity (e.g. a dropped hair dryer) being introduced into the bath OR a person in the bath coming in contact with electricity external to the bath (e.g. a faulty shower). Both are incredibly unlikely but I suppect that, in general, the latter would be the more likely (hence probably favouring not earthing the bath).

Kind Regards, John

 

[ekmdgrf]

It's an interesting point about the path of least resistance being between line and neutral conductors in the hair dryer. I hadn't considered that. I magine in the case of the bar fire, that it is tricy to submerge both ends.

I guess another factor is the exit path from the bath. Is the metal bath covered in insulating paint, which I imagine would have to take the current to the metal around the plug hole?

It would be intersting to measure the resistance of water with different bath products. Although tap water is Chlorinated, so I imagine it's quite conductive. Probably many times more than the human body. I'll have to test that out.

I imagine a body in a bath causes all sorts of interesting resisotors in parallel questions. I feel like ending with another xkcd:

http://www.xkcd.org/356/

 

[bernardgreen]

Indeed, and that's what one would expect. Given that the output of a defibrillator is floating,

It was voltage gradient measured using a two point probe and not the absolute ( relative to ground ) voltage.

As regards to a person in an un-earthed bath touching a live ( due to a fault ) pipe..... if the bath is totally free of any connection to earth ( be it ground or the neutral derived CPC ) then the person would be very un-likely to receive a serious shock. But what if there was a path to earth that was of an impedance low enough for a fatal shock but too high for the 30 mA current (nomina ) needed to trip the RCD. The live voltage would remain on the pipe until the trip current is reached. On this basis it may be preferable to earth the bath ( or bond it to water pipes ) to ensure the cuurrent is high enough to trip the RCD and cut power before the current through the body does serious harm.

As the RCD would not trip the bath water would remain live potential via the person's body. That would be a hazard for anyone who touched the bath or the person in the bath and something that was earthed. They would not be aware the bath and the person in it was at a lethal potential as the person in the bath and holding the pipe would not be showing any signs of being shocked.

 

[JohnW2]

It was voltage gradient measured using a two point probe and not the absolute ( relative to ground ) voltage.

I realise that - hence my comment. Since there is no earth reference in the equation, it's essentially the same as the 'hairdryer in an earth-free bath' scenario. As they demonstrated (and despite what you suggested previously) there will be some voltage field in the water surrounding the turkey (or hairdrier) but not of a dangerously high magnitude.

As regards to a person in an un-earthed bath touching a live ( due to a fault ) pipe..... if the bath is totally free of any connection to earth ( be it ground or the neutral derived CPC ) then the person would be very un-likely to receive a serious shock....

Exactly.

...But what if there was a path to earth that was of an impedance low enough for a fatal shock but too high for the 30 mA current (nomina ) needed to trip the RCD. The live voltage would remain on the pipe until the trip current is reached. On this basis it may be preferable to earth the bath ( or bond it to water pipes ) to ensure the cuurrent is high enough to trip the RCD and cut power before the current through the body does serious harm.

That's all theoretically true, but I think you're scraping the barrel in relation to extraordinarily unlikely scenarios. Indeed, in any situation, one can postulate high impedance paths including a human body(e.g. due to poor contact with the person) resulting in a current through a person which was not quite high enough to result in RCD operation.

The real 'worry'about what you say is that you are suggesting that one should take measures aimed to deliberately increase the current flowing through a person under certain circumstances, purely in order to cause an RCD to operate. Given that RCDs are far from panaceas, and that sufficiently high currents can theoretically result in death during the brief period before an RCD operates, I would personally be very hesitant to suggest deliberately increasing the current through the person during that brief period, even if the well-intentioned idea was to ensure that period was brief (due to RCD operation).

Kind Regards, John

 

[bernardgreen]

I am not suggesting it. I am merely pointing out that it is not a clear cut decision as to whether a bath should have an intentional connection to earth or be fully isolated.