obtusewit
Fri, 21st May 2004, 11:07 AM
I have recently been party to a thread on RC concerning a reef keeper who reported he was being shocked even though he had a ground probe in his tank and he thought his fish had been electrocuted. As an electrical engineer, who frequently gives training and consults on electrical safety, I was absolutely shocked (pardon the pun) at the number of misconceptions reef keepers have concerning electrical systems. I felt that I should post a thread in this forum so I don't have to wear a suit to the funeral home in the near future
I think all of us have suffered that invigorating tingle of an electrical shock, but most people don’t realize how lucky they are to have dodged the fatal bullet. It takes a surprisingly low amount of electrical current to cause serious damage or death. Although voltage plays a part in all of this, it is the flow of current that kills and maims. To give everyone an idea of the effects an electrical current has on the human body, here are the numbers in milliamps (ma). 1 amp = 1000 ma
Current (ma) Reaction
1 Perception level
5 Slight shock felt; not painful but disturbing
6-50 Painful shock; "let-go" range
50-150 Extreme pain, respiratory arrest, severe muscular contraction
1000-4,300 Ventricular fibrillation
10,000+ Cardiac arrest, severe burns and probable death
Anything above the 6 ma “let go” range is considered unsafe. “Let go” current is the current level at which you no longer have control of your muscles and are unable to “let go” of an energized conductor. If you sustain an electrical current of sufficient magnitude flowing through your body for as little as 1 full cycle (at 60 cycles per sec, 1 cycle = .016 secs), it is not a healthy lifestyle.
To give everyone an idea about what a milliamp is. At 120 volts, a 60 watt light bulb draws 500ma, a 250 watt MH draws about 2100 ma, and a 10 watt power head draws 83 ma. In other words, the current flowing through a 60 watt light bulb is 83.3 times what is considered to be a lethal dose.
Circuit breakers-
Circuit breakers operate at a predefined current level. A breaker is designed to clear the fault current above a preset level in 6 cycles, 3 cycles to sense the current and 3 cycles to “clear” (mechanically opening the contacts). Most household receptacles are protected by a 15 amp breaker. If you come into contact with the energized conductor on a 15 amp (15,000ma) circuit, and its current flows through your body, you will die before the breaker trips. There is no question about that, and in all likelihood, the breaker will never trip because your body has sufficient resistance (about 1000 ohms) to limit the current to less than 15 amps. In order for a breaker to trip, it must ‘see’ the current. If the water in your tank isn’t grounded, then power cannot flow to ground. In all liklihood, a short circuit into your tank will not trip the breaker anyway. Because, there is something else at play here, resistance, and I will discuss that later.
GFCI- Ground Fault Circuit Interrupter – Safe at last – NOT
A ground fault circuit interrupter is really a misnomer, it does not trip if there is a ground fault, it trips when the current flowing in the hot and the neutral are unbalanced by more than 6 ma. Safer? Yes. Totally Safe? I think not, and here are the reasons why.
You have just installed a GFCI according to all the instructions and it is absolutely working correctly. You pull that new power head out of the box and stick that bad boy to the back of your tank and plug it in. Water flows, the sediments cloud the tank and you pronounce your accomplishment a resounding success. The next morning your wife decides that that little piece of algae on the glass just has to go, …well, you find her on the floor dead and your power head is still working (whew, that new acro cost a fortune). What happened? Well, the hot wire to the power head has a cut in it. And since there wasn’t any electrical current flow out of the tank because no ground path existed, the GFCI sat there fat, dumb and happy. Until…. your wife was touching a nice solid ground when she became the current path between the hot stuff and ground. UL allows a 200ms (.2 seconds) trip time, about 10 times as long as it takes to sustain some serious physiological effects…Thank go her kness buckled before the GFCI tok that powerhead offline. Now you are now a widower.
Resistance (I told you I would get back to this)
Conductivity is measured in siemens. In seawater, at the standard concentration of S35, at 28 deg C conductivity is 57,015 µS/cm. 1 siemen = 1,000,000 µS. 1 Ohm = 1/(µS * 0.000001) therefore the resistance per cm of seawater = approximately 17.53 ohms. Let’s assume we have a 48 inch wide tank, which would be about 120 cm, then the total resistance from end to end would be 120 x 17.5 = 900 ohms. Powerhead on one end, ground probe in the other corner(we reefers like things out of the way)
Current = Voltage/Resistance = 120/900 = .133 amps = 133 ma. If you have a ground probe in your tank, your GFCI will trip in under 200ms because some current will flow to ground and not back through the neutral, letting the GFCI ‘see’ an unbalance. If there is no ground probe, you might be a ‘deadneck’ when you become the ground probe. Your 15 amp breaker would never ‘see’ 15 amps and trip. Resistance through the water would limit the current to less than the 15 maps
SO what have we learned from any of this
1. A Ground Probe is worthless without a GFCI
2. A ground probe will not protect you if the tank becomes energized
3. A GFCI might not protect you if the tank becomes energized
4. Any electrical equipment in a tank is a potentially dangerous thing
5. Always make you wife scrape algae.
Next weeks topic - Steam turbine power heads, do I need a chiller?
I think all of us have suffered that invigorating tingle of an electrical shock, but most people don’t realize how lucky they are to have dodged the fatal bullet. It takes a surprisingly low amount of electrical current to cause serious damage or death. Although voltage plays a part in all of this, it is the flow of current that kills and maims. To give everyone an idea of the effects an electrical current has on the human body, here are the numbers in milliamps (ma). 1 amp = 1000 ma
Current (ma) Reaction
1 Perception level
5 Slight shock felt; not painful but disturbing
6-50 Painful shock; "let-go" range
50-150 Extreme pain, respiratory arrest, severe muscular contraction
1000-4,300 Ventricular fibrillation
10,000+ Cardiac arrest, severe burns and probable death
Anything above the 6 ma “let go” range is considered unsafe. “Let go” current is the current level at which you no longer have control of your muscles and are unable to “let go” of an energized conductor. If you sustain an electrical current of sufficient magnitude flowing through your body for as little as 1 full cycle (at 60 cycles per sec, 1 cycle = .016 secs), it is not a healthy lifestyle.
To give everyone an idea about what a milliamp is. At 120 volts, a 60 watt light bulb draws 500ma, a 250 watt MH draws about 2100 ma, and a 10 watt power head draws 83 ma. In other words, the current flowing through a 60 watt light bulb is 83.3 times what is considered to be a lethal dose.
Circuit breakers-
Circuit breakers operate at a predefined current level. A breaker is designed to clear the fault current above a preset level in 6 cycles, 3 cycles to sense the current and 3 cycles to “clear” (mechanically opening the contacts). Most household receptacles are protected by a 15 amp breaker. If you come into contact with the energized conductor on a 15 amp (15,000ma) circuit, and its current flows through your body, you will die before the breaker trips. There is no question about that, and in all likelihood, the breaker will never trip because your body has sufficient resistance (about 1000 ohms) to limit the current to less than 15 amps. In order for a breaker to trip, it must ‘see’ the current. If the water in your tank isn’t grounded, then power cannot flow to ground. In all liklihood, a short circuit into your tank will not trip the breaker anyway. Because, there is something else at play here, resistance, and I will discuss that later.
GFCI- Ground Fault Circuit Interrupter – Safe at last – NOT
A ground fault circuit interrupter is really a misnomer, it does not trip if there is a ground fault, it trips when the current flowing in the hot and the neutral are unbalanced by more than 6 ma. Safer? Yes. Totally Safe? I think not, and here are the reasons why.
You have just installed a GFCI according to all the instructions and it is absolutely working correctly. You pull that new power head out of the box and stick that bad boy to the back of your tank and plug it in. Water flows, the sediments cloud the tank and you pronounce your accomplishment a resounding success. The next morning your wife decides that that little piece of algae on the glass just has to go, …well, you find her on the floor dead and your power head is still working (whew, that new acro cost a fortune). What happened? Well, the hot wire to the power head has a cut in it. And since there wasn’t any electrical current flow out of the tank because no ground path existed, the GFCI sat there fat, dumb and happy. Until…. your wife was touching a nice solid ground when she became the current path between the hot stuff and ground. UL allows a 200ms (.2 seconds) trip time, about 10 times as long as it takes to sustain some serious physiological effects…Thank go her kness buckled before the GFCI tok that powerhead offline. Now you are now a widower.
Resistance (I told you I would get back to this)
Conductivity is measured in siemens. In seawater, at the standard concentration of S35, at 28 deg C conductivity is 57,015 µS/cm. 1 siemen = 1,000,000 µS. 1 Ohm = 1/(µS * 0.000001) therefore the resistance per cm of seawater = approximately 17.53 ohms. Let’s assume we have a 48 inch wide tank, which would be about 120 cm, then the total resistance from end to end would be 120 x 17.5 = 900 ohms. Powerhead on one end, ground probe in the other corner(we reefers like things out of the way)
Current = Voltage/Resistance = 120/900 = .133 amps = 133 ma. If you have a ground probe in your tank, your GFCI will trip in under 200ms because some current will flow to ground and not back through the neutral, letting the GFCI ‘see’ an unbalance. If there is no ground probe, you might be a ‘deadneck’ when you become the ground probe. Your 15 amp breaker would never ‘see’ 15 amps and trip. Resistance through the water would limit the current to less than the 15 maps
SO what have we learned from any of this
1. A Ground Probe is worthless without a GFCI
2. A ground probe will not protect you if the tank becomes energized
3. A GFCI might not protect you if the tank becomes energized
4. Any electrical equipment in a tank is a potentially dangerous thing
5. Always make you wife scrape algae.
Next weeks topic - Steam turbine power heads, do I need a chiller?