|Send to a Friend||View / Add Comments|
Co-Authors - Proprietary
Research has been ongoing to determine why people are drowning due to electric shock in freshwater marinas across the nation. In addition to those occurrences where people are known to have been shocked, more drownings may indeed be caused by electric shock than are reported because autopsy results are not able to pinpoint this as the cause.
The freshwater environment is of most concern since salt water's high conductivity allows the majority of ground fault currents to pass harmlessly to the earth ground (this has been previously quantified through a significant amount of research in saltwater). In freshwater, where electrical resistance is high, a person's body represents the low resistance path that the current is seeking to return to ground. Hence, the occurrence of deadly electric shocks to immersed people in freshwater marinas, and none being reported in saltwater.
This research will substantiate the need to monitor the electrical condition of a marina using available marina monitoring tools to reduce the chances for personal injury and death from electrical shock induced by electrical faults in boats and marina electrical systems.
An opportunity recently became available to conduct electrical testing in a large, well-maintained marina facility located on a freshwater reservoir in the southeastern United States. The summary of this freshwater research is presented below. It will demonstrate the following:
Methodology, Test Results, and Analysis:
The boat used for the testing (30 ft express cruiser with twin stern drives) was being supplied with a single 30 amp, 120 volt shore cable. The dock was also capable of supplying 50 amp, 240 volt power. The test sequence involved the following steps:
1. Determine the integrity of the ground connection from the boat to the shore. The instrument used was a Suretest Model ST-1THD.
Ground integrity. Max current (assuming a worst case direct short to ground, and based on 0.72 ohms ground resistance), for 123 volts on this ground leg is 171 amps. Although this ground resistance is higher than normally observed, it should still allow enough fault current to flow to cause a 30 amp breaker to trip instantaneously. Based on the premise that the ground wire must be capable of carrying at least five times the breaker's rated current during a fault, the integrity of the ground wire from the boat ground to the earth ground ashore is acceptable.
Also notable is the difference in ground resistances and voltage drops between measurements taken on the pedestal and on the boat. These indicate that the dock pedestal supply and ground is in better condition than the boat, and that there may be a problem with relatively higher resistances at the shore connection (boat or pedestal side) or on the boat itself.
2. Determine the effects of inducing a known ground fault while simulating loss of ground continuity. To accomplish this, a jumper was installed between the shore cord and the pedestal. This jumper is equipped with a ground lead that can be opened to simulate a loss of ground continuity to the boat. Additionally, a device was installed on the boat to introduce a 5.2 amp ground fault between the boat's hot and ground legs. This simulates a ground fault that is of low enough magnitude so as to not trip any circuit breakers, a condition that can persist unnoticed by boat owners or marina personnel (unless continuous or frequent monitoring routines are in place).
Current flow into the water was measured by clamping the cable with a Hyoki Model 3283 clamp-on ammeter. The meter reading indicated the current that is not coming back on the neutral or ground wires and therefore must be leaking into the water through the boat's ground system via grounded, underwater equipment. The following summarizes the tests and measurements taken.
Analysis: With the ground intact, 98% of the fault current is carried safely off the boat to ground. This provides protection to personnel in the event of a ground fault. With the ground open, 31% of the maximum possible fault current enters the water seeking earth ground. This creates an electric field that is discussed in subsequent tests.
Analysis: Note that the sum of the two faulted voltage drops above (c + d) equals the total no-fault voltage measured in part a. This was as expected. The 84 v measured in part c above represents the voltage on any grounded underwater metal component on the boat. In other words, touching any of these metal parts directly would most likely result in a fatal shock if the person is grounded through the water. The 1.6 amp fault current measured indicates that the water resistance in this part of the freshwater reservoir is approx 50 ohms (using Ohm's Law: 84 volts/1.6 amps = 52.5 ohms).
A confirmation calculation was made to ensure that the 0.069 amp current leaking into the water with the ground connected correlated with the total fault current (5.2 amps) and the measured ground resistance (0.72 ohms). The total fault current, with the ground connected, should split in proportion to the ground resistance and water resistance.
Amps leaking into water = 5.2 amps x
0.72 ohm ___________________ = 0.073 amps
This calculation validates our ground resistance and current measurements.
3. Determine the presence and extent of any electric fields in the water generated by the ground fault. Examine any affect this field might have on neighboring boats and nearby dock structures. To accomplish this, a ground fault was induced and the ground opened as above (simulating the two conditions necessary to force current flow into the water and establish an electric field). A copper probe was immersed in the water in various locations around the boat and the voltage between this probe and the dock ground was measured (with the fault induced and normal ground open). The voltage on a nearby dock adjusting cable (attached to the bottom) was also measured.
Analysis: An electric field was measured at and in the vicinity of the ground-faulted boat. It was strongest near the test boat and tapered off with distance. The exception to this was the field measurement made on the boat across the dock, directly astern of the test vessel. This has to be further examined to explain the finding. Since the depth of the water was reported to be approx. 18 ft, these measurements showed that the electric field was strong enough directly behind the boat (2.3 - 2.8 volts per foot) to be potentially lethal to a person entering the water in this field (2 volts per foot generally considered to be lethal). In other areas further from the boat, the effects of the field would likely be felt as a shock or tingle while in the water. The resulting panic from this sensation could cause a person to sustain injuries or death from the inability to swim safely out of danger.
The voltage measured on the dock adjusting cable indicates that some of the ground fault current in the water is seeking this path back to earth ground. It is possible that a person touching this cable could feel a shock or tingle if they are also grounded to the dock.
Analysis: On all boats clamped during the faulted condition, current increased in the shore cables. This again represents current escaping the water and returning to ground via the dock ground system and indicates how ground fault currents can travel in the marina when the ground on the faulted boat is lost. Being in this current path between boats is also a hazard.
We have shown that when a ground fault is induced in a purely freshwater marina, and the ground on the boat is disabled, potentially lethal electric fields are established near and around the affected boat. The voltage potential of other dock structures are also elevated, presenting further risk of injury.
Actions can be taken as follows to minimize the possibility of the two necessary conditions (ground fault and reduction in ground integrity) existing simultaneously:
1. NFPA 303 requires an annual verification of the integrity of the grounds in marinas. This test may be performed more often by trained marina personnel to ensure the safety afforded by a proper ground system is maintained.
2. Equipment is available to monitor docks continuously for ground faults (both dock and boat related). With a monitoring system installed, the marina operator would be alerted immediately whenever a ground fault occurs. Operators can then take swift action to locate and isolate the fault using the fault monitoring system and logical isolation procedure.
By periodically ensuring the ground system is in good condition, and by proactively eliminating ground fault conditions, the marina operator will be presenting the safest possible environment for marina clients and staff.
|[ Back to Top ]|