Northeast Fisheries Science Center Reference Document 06-26
Protection against Electric Shock
in Laboratory Sea-Water
Systems
by James M. Crossen, Paul S. Galtsoff, and Jon A. Gibson (ed.)
National
Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole MA
02543-1026
Print
publication date 1966;
web version posted November 28, 2006
Citation: Crossen JM, Galtsoff PS, Gibson JA (ed.).
2006. Protection against electric shock in laboratory sea-water systems.
US Dep Commer, Northeast Fish Sci Cent Ref Doc 06-26; 20 p.
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Foreword: In January 2006, Jim Crossen brought this
paper into the Editorial Office of the Northeast Fisheries Science Center
(NEFSC). Jim indicated that he, as an employee of the Woods Hole
Biological Laboratory of the Bureau of Commercial Fisheries (forerunner
of the National Marine Fisheries Service), had authored the paper in
1966, but had not placed it in the Woods Hole Laboratory Reference
Document series at that time. He felt that the information
in the paper had some lasting value, and asked that the paper be handled
in a manner to assure its retention by the NEFSC. Accordingly,
the Editorial Office has issued this paper as an “unpublished paper” which
will be included in the listing of 2006 reports prepared by NEFSC authors,
and will be available through that listing on the NEFSC’s publications
webpage (www.nefsc.noaa.gov/nefsc/
publications/lists/lists.htm).
Workplace safety has always been a serious concern of organizations
and their members, although one could argue that the subject has been
taken more seriously in the 40 years between when this paper was prepared
and when it was issued. What is not arguable is that workplace
safety has been far more institutionalized in those four decades. Today,
there are laws, regulations, agencies, and specialized/ dedicated personnel
all addressing aspects of workplace safety. Therein lies the importance
of, and interest in, this paper: it largely reflects the “pre-institutionalized” approach
to workplace safety. Individual initiative, investigation, fabrication,
and evaluation were the means by which a potentially serious safety problem
at the Woods Hole Laboratory was solved.
Jim Crossen authored all of the paper except for the section on “Fouling
Analysis.” Paul Galtsoff, the Director Emeritus of the Woods
Hole Laboratory, performed such analysis and wrote the associated section. Unfortunately,
Tables 6 and 7 of the paper have been lost through the passage of time.
I have made some changes to the paper, and they need to be listed in
order to differentiate between the paper as originally prepared and as
issued. I prepared the title page, foreword, table of contents,
and figure captions. In the original paper, most of the pages were
not numbered; in the new table of contents, the implied page numbers
of the various components of the paper are indicated by numbers within
squared brackets. In the original paper, there were no figure captions
associated with the figure images; I prepared those captions based upon
the text descriptions and language. Additionally, at some point
subsequent to 1966, someone had reviewed the paper and had scrawled a
few comments directly onto the paper in pencil and ink. I removed
those comments.
Jon A. Gibson
Biological Sciences Editor
Northeast Fisheries Science Center
April 19, 2006
Abstract
An electrical shock hazard can exist to personnel in modern sea-water
systems which use hard rubber and synthetic piping. A method, non-toxic
to marine life, is described herein which safeguards personnel against
electric shock by the insertion of grounded platinized titanium electrodes
in the system.
The Bureau of Commercial Fisheries Biological Laboratory at Woods Hole,
Massachusetts, maintains a salt water supply system with a capacity of
76,000 gallons. This system provides a water supply for the main laboratory
and for the public aquarium. A breakdown of the total water capacity
is given in Table 1. A diagram of the salt water system at Woods Hole
is shown in Figure 1.
Sea water supply systems for aquaria which have in the past used metallic
piping, not utilize non-corrosive hard rubber and poly-vinyl-chloride
(PVC) type plastics. While the latter has eliminated the problems of
toxicity to fish inherent in the combination of salt water and metals,
it has presented a potential shock hazard to personnel.
Modern sea water systems such as that at Woods Hole which supply experimental
aquaria are carefully designed to avoid the use of metals. Most metals
exposed to salt water corrode to some degree due to galvanic action resulting
in the liberation of ions which are toxic to some marine organisms.
Chemically inert materials such as hard rubber, polyurethane, poly-vinyl-chloride,
epoxy base sealers, vulcanite, ebonite, wood covered with black ashphaltum
are non-conductors of electricity.
The Woods Hole system utilizes “Uscolite,” a synthetic rubber
material (styrene acrylonitriles – buitadene copolymer) for the
piping. The iron reservoir tank is coated with “Gunite,” and
insulating cement. Under certain conditions, that is, when water is not
being pumped into the reservoir tank (see Figure 2) and is not being
returned to the harbor through drains, a floating system exists without
an electrical ground return.
INTRODUCTION
Electrical Shock Incident
The following is an account of an electrical shock problem which took
place in the main laboratory of the Woods Hole Biological Station.
Electrical shocks were being experienced from a tank in Room 112 (see
Figure 3A and 3B). Upon investigating the source of trouble it was found
that a potential of 50 volts alternating current (50 V.A.C.) existed between
the salt water tank (A) and ground (B). A potential of 25 V.A.C. was found
in the same supply line in the tank room (C). Tracing the voltage in the
opposite direction isolated the source to Room 111, point (D). A defective
immersion type heater element had corroded at a brazed seam allowing water
to penetrate the housing. The heater had not been properly grounded through
a low resistance, therefore an electric current entered the salt water
line at point (D). When water was not being drawn into the tank in Room
11, the circuit was broken and no current flowed.
Because some water was flowing from the tank (A) in Room 112 to the drain
(electric ground), a low resistance path equal to approximately 200 ohms
existed. Therefore, the electric current passing through the shocked person’s
dry body was not dangerous to life (see Table 2). Normal dry skin affords
a comparatively high resistance which allows only a small amount of current
to flow through the body. However, the resistance of wet skin allows thousands
of times more current to flow. The path of electric current through the
body is vitally important. Current flowing from one finger to another on
the same hand would not pass through vital organs, such as a current passing
from one hand to the other hand.
Safeguard Methods
A study was initiated to determine a method of effectively grounding any
electricity which may accidentally become exposed to a part of the system.
In order to safeguard against shock hazards the following should be done:
1. All electrical tools, appliances, instruments, especially those used
in experimental tanks, i.e., immersion heaters, stirrers, metal cooling
coils, electrodes for sensing oxygen, salinity, pH, etc., must be properly
grounded. If the insulation in electrical equipment should break down,
the frame and other metal parts of that equipment becomes energized. If
a properly grounded wire is connected to the frame, the electrical current
will follow the wire which is the path of least resistance.
Determine whether the electrical wiring is the three wire type or two
wire type. New buildings, in accordance with the national electric code,
have three wires, one of which is a ground to drain off potential shocks.
This type if compatible with appliances having three prong plugs.
In a two wire system install a single conductor number 18 copper wire
to all appliances and attach it to a screw on the appliance. Then attach
the other end of the ground wire to the grounded outlet box. Caution: Attach
the ground wire before inserting the plug in the outlet.
2. A method must be used to ground the salt water system without the use
of toxic metals which would defeat the intended purpose of plastics. A
study was made of materials which would meet the following requirements:
(1) High corrosion resistance.
(2) Low electrical contact resistance.
(3) Freedom from fouling.
Graphite, titanium, and platinized titanium were selected after making
a study of available materials. All of these materials are at the most
noble (least corrosive) end of the galvanic series of Table 5 and Table 6 (missing).
Samples of the above were placed in isolated tanks containing fish for
a period of over three months. No toxic efforts to the fish were apparent
and no corrosion or pitting of the materials occurred.
ELECTRODE INSTALLATION
The 4 inch synthetic rubber pipe used throughout the Woods Hole system
is tapped on the sidewall (see Figure 4) to accommodate stopcocks. A foam
rubber material covers the piping for the purpose of eliminating water
drippage. The tapped holes are 5/8 of an inch in diameter and are sealed
with plastic plugs where stopcocks are not used.
The graphite rod used was National Carbon grade AGSR, #P2718. It was 5/16
of an inch x 6 inches and has an electrical resistivity of 8.40 x 10-4 ohms/cm.
The titanium rod was ¼ of an inch x 6 inches and has an electrical
resistiviyy of 5.48 x 10-5 ohms/cm. The plastic plugs were machined
to accept the rods and an epoxy resin was used to provide a watertight
seal (see Figure 5).
The electrodes were inserted in the plastic supply lines and connected
through a #12AWG copper wire and brass clamp to the copper pipe compressed
air system which runs parallel to the sea water supply lines throughout
the laboratory and aquarium. In other systems connection could be made
to metallic water pipes, steam pipes, or a low resistance source to the
earth. Paint and other insulating material should be removed from the surface
before attaching the ground clamp.
When using electrical apparatus in experimental tanks, portable probes
(see Figure 6) should be suspended into the water and connected through
a copper wire and clamp to a suitable ground.
ELECTRODE MAINTENANCE
Regular checks should be made annually to determine the contact resistance
between electrodes. If the electrodes have become fouled, they should be
thoroughly cleansed before being reinserted. A dilute hydrochloric acid
(1 part acid, 3 parts water) is an effective solvent when necessary. Titanium
and platinized titanium installed in the Woods Hole system since 1963 and
1965 respectively have shown no sign of corrosion or pitting. Only a small
amount of fouling was present and it was easily brushed clean.
FOULING ANALYSIS
Examination of two electric probes removed from the Woods Hole Laboratory
sea-water system on 15 March 1966.
General observation: Fouling is light on both probes. It consists of gray
material adhering to metal. No live organisms are visible to the naked
eye.
Probe 1: Flat titanium strip covered in places with flaky material and
loose sediment. Only about 10-15% of probe surface covered with fouling.
Microscopic examination reveals the following: completely, transparent,
colorless scales with dark black particles embedded or attached to them.
The black material is in form of irregular stars resembling deposit of
sulfides of some heavy metal. Loose graying sediment consists primarily
of a mass of sponge spicules in places corresponding to the configuration
of small bodies of live sponges. Spicules of calcareous sponges of the
type of Grantia and many loose spicules of silicous sponges are present
in abundance. They are covered with organic detritus and give support to
numerous sedentary infusoria, probably Vorticella patellina which appear
to be in healthy condition. Occasional diatoms of the type of Pleurosigma
and short colonies of Melosira were present. Two small live annelids were
found: one of the Terebellidae and the other of Sabellidae. Both specimens
too small for species identification. They were building tubes of loose
sediment particles and sponge spicules. Small, not identifiable flagellates
and infusoria, were occasionally seen.
Probe 2: The platinized titanium rod was covered with light material similar
in appearance to that of probe 1. About 60% of probe surface was covered
with fouling.
Fouling consisted primarily of loose sponge spicules of calcareous and
siliceous sponges mixed with organic detritus. Few bottom diatoms of Pleurosigma
type were present. Live Vorticellae were present.
No annelids were found and the transparent flakes with black particles
were absent.
DISCUSSION
An account is given of methods and materials used in the Woods Hole sea-water
system to safeguard personnel and fish against electric shock. Table 7
is a summary of the electrical resistivity of electrodes inserted in the
sea-water supply lines.
The graphite rods installed in the pipe lines for well over a year have
a very low contact resistance which is in part due to the material porosity.
While this porosity provides a good ground, it also allows water to reach
the clamp and copper wire causing corrosion. A conductive epoxy resin (Eccobond
60) which has a bulk resistivity of 50 ohms/cm and a resistance of less
than 1 ohm/3 mils was used to seal the exposed surface of the graphite.
This method of sealing is not satisfactory since some corrosion continues
to exist.
The titanium rods inserted in the system in February, 1963 have a high
contact resistance compared to graphite. Examination of the titanium after
being exposed to the salt water line for a two year period showed no apparent
corrosion or pitting. However, because it was desirable to improve the
contact resistance, samples of platinized titanium were obtained and tested.
Titanium by itself has a high electrical resistance because of a tenacious
oxide film which forms on the surface preventing metallic ions from migrating
into the sea-water. This film, however, also is responsible for the unusually
high corrosion resistance of the titanium.
Therefore, by combining the two metals, with a thin coating of platinum
on titanium, a low electrical contact resistance is realized without impairing
titanium’s corrosion resistance.
Platinized titanium rods have been installed in the Woods Hole system
for over one year. They are inserted in the supply pipe lines at approximately
every 30 feet and one in each room of the main laboratory’s first
floor.
LITERATURE CITED
Holmes, John F. 1966. Wide Range Flow Meter for Oceanographic Measurements.
Meyer-Waarden, P.F. 1957. Electric Fishing, FAO Fisheries Study No. 7.
Redfield, Alfred. 1948. Characteristics of Sea Water, Corrosion Handbook,
Edited by Uhlig. John Wiley and Son, pp 1111-1122.
_____. 1961. National Industrial Carbon and Graphite Catalog S-5950, National
Carbon Company, 270 Park Avenue, N.Y. 17.
_____. ----. Occupational Safety Aid MP-8-0 and MP-9-9, U.S. Department
of Labor, Bureau of Labor Standards, Division of Safety Standards and Services,
Washington 25, D.C.
_____. ----. Platinized titanium, Englehard Industries, Newark, N.J.
_____. ----. Titanium metals Handbook for the Chemical Processor, Titanium
Metals Corporation of America, 233 Broadway, New York 7, N.Y.
