lethal voltage

[nizak]

At what point does the voltage that may be present in pool water become deadly? I have heard people in the pool business say that 1 or 2 volts is merely a nuisance. I realize that many factors (persons age, possible medical conditions,pool water composition, etc.)can contribute to a fatality.

 

[Redcliffe]

http://hypertextbook.com/facts/2000/JackHsu.shtml

 

[GoldDigger]

Two concerns:

1. As indicated in the article, what really matters is the current and the path it follows. But you can make some good approximations of the resistance of the body when immersed in water and with the skin fully hydrated.
2. The thing that drives the current is not the "voltage in the water."
It is specifically the voltage gradient from one point in the water to another, given that a body is joining those two points. That can be high or low in any particular part of the water, based on the water's conductivity and the geometry of the energized parts.

Tapatalk!

 

[hurk27]

I have read studies that said 2-3 volts can cause a person to not be able to pull themselves out of the water, even one volt on a person immersed in water, specially treated with very conductive chlorine can be very painful, of course this can vary from person to person because of the difference in internal body resistance each person has, but a person immersed in water will have a very low skin resistance which can lead to much higher currents flowing through the body, 4-5 volts can cause a person to loose all muscle control and prevent them from keeping their head above the water, a little more and it can affect the heart.

 

[hurk27]

Two concerns:

1. As indicated in the article, what really matters is the current and the path it follows. But you can make some good approximations of the resistance of the body when immersed in water and with the skin fully hydrated.
2. The thing that drives the current is not the "voltage in the water."
It is specifically the voltage gradient from one point in the water to another, given that a body is joining those two points. That can be high or low in any particular part of the water, based on the water's conductivity and the geometry of the energized parts.

Tapatalk!

note to your #2, problems with pools with no EPB and or metal docks or boat lifts are the fact that these items are many times bonded to the service grounding system, this grounding system is then also bonded to the service neutral which in many cases will have a voltage drop or the MGN ahead of the service transformer can have a voltage drop, this voltage drop will cause the grounding system to be a few volts above the earth potential which is where the problem lies, a concrete pool with no EPB bonding the concrete will have it's water at Earth potential and if a ladder or pool cover is at the potential of the service grounding because of being bonded to it, a person might not be able to pull them self out of the water, the same goes for docks and boat lifts because the lake or river will be at the Earth potential, but the boat dock or boat lift is at the potential of the service grounding, generally the voltage gradient across the shells in water from a voltage drop like this will be very low unless there is a much higher voltage drop like there would be with a loss ed service neutral connection.

In having voltage gradients in the water mostly come from when you have a fault or bad wiring and there were no proper EGC to open a OCPD which results in the boat or dock or boat lift being at a 120 volt potential to the water, we then have a very high gradient even in small areas around such equipment, because of the conductivity of the water these shells are very much smaller then the shells around a ground rod in Earth, so a person has a much more likelihood of of receiving a much higher voltage across a shell, this is why it can be so much more dangerous.

But one point I would like to make, many have the thinking that this voltage can travel a long ways in water, this is a myth, like the sphere of influence around a ground rod in Earth, the SOI around an injection point in water is also much smaller than it would be around a ground rod in earth, while a ground rod in Earth in average resistance soil is about 26' which is the point that 100% of the injection voltage would have dropped, in water this will be much smaller like you said depending upon how conductive the water is, the lower the conductivity of the water the smaller the SOI, this is why salt water has such a small SOI that the danger if receiving a shock from one side of the body to the other is not very likely, most electrocutions in salt water are when a swimmer is getting in or out of the water and grabs a live boat or metal dock or other type of electrical equipment that has a difference of potential to Earth.

 

[don_resqcapt19]

I think that one of the issues with voltage on the water and people in the water is that the wet skin is a more conductive path than the water itself. That causes more current flow over the length of the body than over the same distance in the water itself.

 

[GoldDigger]

The central :angel: issue is that the conductivity of the electrolyte environment inside the body is even greater than that of the wet skin. The skin is just the entry point.

Tapatalk!

 

[Jon456]

This article is a good read (albeit it starts off recounting some very tragic events) about the danger of Electric Shock Drowning and the hazards of AC leakage into water, particularly fresh water.

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

The owner of the facility where I work bought a 60' yacht, and a routine survey by the marina's harbormaster discovered that it was leaking a considerable amount of current from the 240V shore power connection into the water, on the order of 1.5 to 2A. If his grandchildren went swimming off this boat, they very likely would have died!

We hired a marine electrician who specializes in this type of fault, and he and I installed an isolation transformer into the yacht to make it safe. I find it baffling that there isn't some regulatory requirement to have these in all boats capable of shore power connection. Here are some photos of the installation for those interested...

This is the frame I fabricated for suspending the isolation transformer from joists within the bow hold:

This is the isolation transformer mounted in the frame. The transformer weighs over 250 lbs! Note that the transformer is electrically isolated from the frame by plastic spacers & bushings:

These are the ELCI breakers for each of the two shore power connections (120V/30A & 208V-240V/50A):

This is the transformer input selector and the volt-amp-meters/alarms for input and output:

 

[kwired]

This article is a good read (albeit it starts off recounting some very tragic events) about the danger of Electric Shock Drowning and the hazards of AC leakage into water, particularly fresh water.

http://www.boatus.com/seaworthy/magazine/2013/july/electric-shock-drowning-explained.asp

The owner of the facility where I work bought a 60' yacht, and a routine survey by the marina's harbormaster discovered that it was leaking a considerable amount of current from the 240V shore power connection into the water, on the order of 1.5 to 2A. If his grandchildren went swimming off this boat, they very likely would have died!

We hired a marine electrician who specializes in this type of fault, and he and I installed an isolation transformer into the yacht to make it safe. I find it baffling that there isn't some regulatory requirement to have these in all boats capable of shore power connection. Here are some photos of the installation for those interested...

This is the frame I fabricated for suspending the isolation transformer from joists within the bow hold:

This is the isolation transformer mounted in the frame. The transformer weighs over 250 lbs! Note that the transformer is electrically isolated from the frame by plastic spacers & bushings:

These are the ELCI breakers for each of the two shore power connections (120V/30A & 208V-240V/50A):

This is the transformer input selector and the volt-amp-meters/alarms for input and output:

Can you give us a schematic of how that transformer is wired? If you connect the frame, hull, all other metal objects to the shore power EGC, you still haven't eliminated voltage drop on the service neutral, the POCO's MGN, or any other miswiring that would cause a voltage rise on the incoming EGC.

 

[Jon456]

Can you give us a schematic of how that transformer is wired? If you connect the frame, hull, all other metal objects to the shore power EGC, you still haven't eliminated voltage drop on the service neutral, the POCO's MGN, or any other miswiring that would cause a voltage rise on the incoming EGC.

You are correct. In fact, before we installed the isolation transformer, I measured current flowing into the yacht on the shore power EGC, even after disconnecting the shore power line and neutral wires.

The way that the isolation transformer makes the boat safe is by isolating ALL shore electrical connections from the boat's wiring and bonded metal fixtures; this includes isolating the shore power's EGC. As you can see in the following diagram, the shore EGC is connected only to the isolation transformer's internal shield; that shield creates a electrical barrier between the primary and secondary windings of the transformer preventing any internal fault in the primary windings from leaking to the secondary windings (and thus to the boat), while also tripping the shore power OCPD/ELCI. The boat's neutral and EGC are both connected to the isolation transformer's case, which derives it's zero potential from a center tap on the transformer's secondary wiring (not shown in the diagram).

Note that the diagram shows a 1:1 isolation transformer. The one we installed was custom-build by Charles Marine Group to our specifications to allow multiple shore power inputs (240V, 208V, and 120V) while outputting 240/120V for the boat's systems.

 

[kwired]

You are correct. In fact, before we installed the isolation transformer, I measured current flowing into the yacht on the shore power EGC, even after disconnecting the shore power line and neutral wires.

The way that the isolation transformer makes the boat safe is by isolating ALL shore electrical connections from the boat's wiring and bonded metal fixtures; this includes isolating the shore power's EGC. As you can see in the following diagram, the shore EGC is connected only to the isolation transformer's internal shield; that shield creates a electrical barrier between the primary and secondary windings of the transformer preventing any internal fault in the primary windings from leaking to the secondary windings (and thus to the boat), while also tripping the shore power OCPD/ELCI. The boat's neutral and EGC are both connected to the isolation transformer's case, which derives it's zero potential from a center tap on the transformer's secondary wiring (not shown in the diagram).

Note that the diagram shows a 1:1 isolation transformer. The one we installed was custom-build by Charles Marine Group to our specifications to allow multiple shore power inputs (240V, 208V, and 120V) while outputting 240/120V for the boat's systems.

Something I never get around (while working anyway) is marina applications, but I guess this works well here because the boat itself would not be covered by NEC but the shore power is. If this were just about anything else you would have to bond the "grounded conductor" of the secondary to the grounding electrode system of the supply and ultimately it would be bonded indirectly to every grounded/grounding conductor on just about the entire continent that is supplied by a utility company.

You very well could/should have a grounding electode at the shore power location, but the sphere of influence of that is about all there would be any concern over and chances are the soil is fairly wet and conductive and this sphere of influence is probably not all that large.

 

[kwired]

I have heard of people using similar isolation transformers at agricultural buildings, particularly dairy barns where there were "stray voltage" issues, but have never seen how it could be NEC compliant to do so as the general rules require the before mentioned bonding between primary and secondary of separately derived systems which would not eliminate stray voltages coming from something like voltage drop on the POCO's MGN.

 

[don_resqcapt19]

You can create a totally isolated secondary on an SDS if you install the transformer in a manner that will permit you to apply 250.110(1) to the transformer enclosure.

 

[kwired]

You can create a totally isolated secondary on an SDS if you install the transformer in a manner that will permit you to apply 250.110(1) to the transformer enclosure.

Thanks for that information - I can see that giving us a start, but if the secondary has a grounded conductor, how can you get around not connecting the secondary grounded conductor to the grounding electrode system?

 

[fmtjfw]

Note: the children (actually anyone) can still be shocked by leakage from other boats, and electrical wiring failures.

In a law to come into effect in WV within a month or so, swimming within 100 feet or so of a marina with shore power is forbidden.

 

[kwired]

Note: the children (actually anyone) can still be shocked by leakage from other boats, and electrical wiring failures.

In a law to come into effect in WV within a month or so, swimming within 100 feet or so of a marina with shore power is forbidden.

leakage from other boats is not that much of a problem. A boat in the water that has a leakage problem will have a voltage gradient around it that drops off pretty fast, only those that enter this small zone are the ones in danger, and it is because the zone is small that you are able to reach across points of different potential. If that zone were large you would not be able to reach across as much potential without help from other conductive objects - like contacting two boats at the same time or something like that, which could be a problem even if you are a passenger in either boat and not a swimmer.

The biggest issues I am aware of at docks and marinas is the EGC of any electrical equipment located there is ultimately connected to the grounded service conductor at some point and is also connected to the utility MGN at some point effectively making it operate at any imposed voltage drop above true ground. We then have the lake which is true ground and a low resistance and being in the water generally decreases the resistance of the body so a small voltage you may not feel on dry ground conducts more current and easily at a lethal level through your body when in that situation.

 

[don_resqcapt19]

Thanks for that information - I can see that giving us a start, but if the secondary has a grounded conductor, how can you get around not connecting the secondary grounded conductor to the grounding electrode system?

The transformer needs to be its own structure and supplied by a feeder. You provide and install a grounding electrode system at that structure but only connect it to XO on the secondary. The metallic enclosure has no connection to any EGC, GEC or bonding conductor.

 

[don_resqcapt19]

555.3 which was new for the 2011 code should help address the issue of shore power leakage current, but does nothing to help with the problem of elevated neutral to earth (water) voltages that will exist on everything that is connected to the electrical grounding system.