Divers have unique applications and conditions under which they interface with electrical power. Fortunately there has only been one death in the Navy's history related to underwater electrical shock. The accident occurred in a 10-foot (3m) deep open training tank at the Deep Sea Diving School, Washington Navy Yard in December 1943. The victim was a student engaged in underwater welding. He wore a Navy Mark V deepsea helmet adapted for shallow-water diving, a swimsuit, and new rubber gloves, but no footwear. Shortly after asking topside to "make it hot," the diver collapsed.

Although he was hauled quickly to the surface and treated by a doctor almost immediately, all attempts at resuscitation failed. The cause of death was determined to be by electrocution. Apparently the victim's bare knee was touching the grounded metal work bench at the same time that the live electrode inadvertently struck the helmet's breastplate, making the diver part of the electrical circuit between the positive and negative poles of the work bench and electrode. Even with this unfortunate death, underwater welding remains a relatively safe task when proper precautions and personal protection are adhered to.

Proper Precautions
To determine proper precautions and personal protection, we need to understand the safe current points and exposure times for the human body, the body resistance of a wet diver, and Ohm's Law. The AODC Code of Practice for the Safe Use of Electricity Under Water (adopted as guidelines for the US Navy in 1987) established safe body current for DC voltage at 40ma (milliamps). Exposure to currents above these limits can create ventricular fibrillation, a disorganized, useless contraction of the heart. The probability of fibrillation occurring increases as current flow through the chest increases. Very little research has been conducted on underwater electrical shock. Although a vast amount of research has been conducted in air, the data is only partially applicable to the hazards faced by a diver using or working near electrical equipment.

There are two basic differences between using electrical equipment in air and using it in water. First, air acts as a relatively strong insulator and thus protects the individual from shock unless he comes in contact with the fault and becomes part of the electrical circuit.

Second, in a dry environment a substantial measure of protection is provided by the skin, which has impedance much higher than internal body resistance. Air in a dry environment provides a resistance of approximately one mega-ohm. A dry body in air has a resistance limb-to-limb of approximately 3,500 ohms. When the body is immersed in water (fresh or salt), the resistance of the skin is lowered. Since seawater is a relatively good conductor, the diver does not have to be in contact with the fault to become part of the electrical circuit. Seawater has a resistance of approximately one ohm. A wet diver has a limb-to-limb resistance of approximately 500 ohms for voltages over 50, and 750 ohms for voltages less than 50.

The only exception to these values are front-to-back of the chest area, where resistance is 100 ohm regardless of voltage. These resistance values are based on a 150-pound person; resistance will vary with the person's weight.

The effects of electrical currents on body tissues are not dependent on whether the source is alternating current (AC) or direct current (DC). However, the mode of current entry and exit of the body (entering and leaving through an appendage, as opposed to the chest cavity), along with the current density, must be considered. These variables, as well as being in an underwater environment, will cause changes in the body's resistance to electricity.

For example, if the current is entering and leaving through the leg, the diver may experience muscle contractions that impair leg mobility. But if that same current enters and leaves through the chest cavity, the diver may experience muscle contractions that cause asphyxiation or ventricular fibrillation, resulting in death.

The susceptibility of the body to shock is proportional to the current entering the body. According to Ohm's Law, the current is determined by the voltage and the circuit resistance (I=E/R, where I=current, E=voltage, and R=resistance). Therefore, the current entering the body would be determined by the body resistance, since voltage produced at any point in a circuit is constant.

If we take the case of the student diver accident victim, we know that he was welding, so the voltage would have been on the average of 29vdc. The current path was through the chest area to the knee, so the body resistance would have been somewhere between 100 ohms and 750 ohms, most likely around 300 ohms.

Using Ohm's Law, we can determine the current flow through the body by dividing the voltage (29vdc) by the body resistance (300ohms), resulting in a current of 97 ma, more than twice the safe current limit, creating ventricular fibrillation.

This was an unusual circumstance where the current path was through the chest. It is not uncommon for a diver/welder to be shocked while welding, but the current path is typically limb-to-limb where we know the body resistance is 750 ohms. Using Ohm's Law, the current flow limb-to-limb is 38ma, which is within the safe current limit.

Alternating current is more dangerous than direct current because of the more severe muscular contractions it will cause. Alternating current of 60 Hz is the frequency considered most dangerous to the human body. Above 60 Hz, the degree of muscular contraction decreases and the possibility of respiratory arrest and ventricular fibrillation are reduced. However, as the frequency increases, heat is produced and the possibilities of serious burns are increased.

With 10ma of AC being the maximum safe limit for a diver, and limb-to-limb resistance of 750 ohms, Ohm's Law shows that the maximum safe AC voltage is 7.5Vac RMS. A voltage of 60 Hz AC is not only dangerous with direct contact but also with an underwater electrical field. A field of 2Vac/foot in saltwater could produce paralysis, and as little as 0.2Vac/foot could produce a chest current flow of five ma.

In 2001, five young boys jumped from a pier into a lake in Manassas, Virginia. Immediately after hitting the water, one of the five boys complained that he could not move his arms and legs and subsequently drowned. Upon investigation it was found that a small boat tied to the pier had a battery charger on and the AC power cord was drooped in the water. A weak spot in the power cord insulation would have created an electrical field which, when the boy entered the water at just the wrong orientation, caused paralysis. Most likely the other four boys were not affected due to their body fat percentages and the orientation of their bodies to the electrical field.

The most dangerous situation for a potential electrical hazard is when an individual is only partially submerged in water, such as workers on the wet deck of a vessel or in a tidal zone. In this situation, the water is the earth ground and the individual handling electrical equipment would be considered an electrode above earth ground and will become part of the circuit if a fault is present.

Requirements for Hazardous Shock
Three actions are required for a hazardous electrical shock to happen. First, the circuit current must be above 40ma DC or 10ma AC for more than 20 seconds. Second, there must be a fault in the electrical system. Third, the individual must be exposed to the fault.

The best method for preventing a hazardous situation is to isolate the individual from any potential fault. One of the more common methods is the use of ground fault interrupters (GFI). Article 100 of the National Electrical Code defines a GFI as, "A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds some predetermined value that is less than that required to operate the over-current protective device of the supply circuit."

The Naval Medical Command, AODC, and International Electrotechnical Commission 479 have determined that the safe point for current through the human body is 500ma for AC circuits and 570ma for DC circuits, for periods not greater than 20 seconds. Based on this safe point, the AODC recommends that GFIs have a trip current not greater than 30 ma, and a trip time of 20 seconds or less. Most off-the-shelf GFIs do not meet these requirements, and provide nothing more than a facade underwater.

Ship and pier power use a variety of power sources that may or may not reference the same ground point, thereby making it difficult to predict current flows, and thus creating potential electric shock hazards.

Since dive stations may not know the type of power source they are tying into, the AODC recommends that an isolation transformer be placed between the power source and the GFI and a new reference ground be established. If a GFI trips, the equipment hooked-up to it should be un-hooked before the GFI is reset. If the equipment has a fault and the GFI is reset with the equipment hooked-up, the fault will be exposed to full power for a period greater than 20 seconds.

Beware of Disassociated Hydrogen and Oxygen
There is one other safety concern that needs to be addressed. Hydrogen and oxygen will become disassociated from the water molecule in the form of gas with as little as three volts being discharged through the water. This is more of a concern during underwater welding applications, since the discharge of electricity through the water is for extended periods of time and the disassociated hydrogen and oxygen gas molecules can collect in an overhead entrapment.

By themselves, these gas molecules are harmless. However, if they are combined again they become an explosive, and if a substantial amount of gas has collected or significant amounts of hydrocarbons are present, the explosion could be deadly. UW

Robert Murray manages the Naval Special Warfare SEAL Support Programs for Naval Sea Systems Command at the Washington Navy Yard.