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Earthing Rods and Electrical Stuff
---------- Forwarded message ----------
From: "/PN=Steven.S.AMOR/OU=ENG4/O=SHELL AUSTRALIA/"@shell.otc.au
Date: 19 Dec 96 17:57:12 +1100
Subject: Earthing Rods and Electrical Stuff
To: Aquatic-Plants at actwin_com (Receipt Notification Requested)
Just before this thread ends (hopefully),
As George says, Earthing Rods in the tank ARE a good idea for the following
reason. In Australia (and many countries around the world) we have MEN
(Multiple Earth Connections). It is the supply authorities' regulations that
each sub circuit is earthed individually (as opposed to say Peru, where I know
that they have 'floating' Neutrals).
The supply authority themselves earth the neutral conductor (3 phases plus
neutral don't forget - one phase and neutral (ie earth) gives 240V AC which is
our supply).
Therefore, if you have a 240 or 110V AC heater in your aquarium and a fault
occurs in the insulation of the ACTIVE phase only then the heater will run fine
- the only way for it to get back to Earth is through the heating elements. If
you put your hand in the tank however, BANG! you become a very nice conductor
with your wet hands and bare feet, straight through you to earth. Since the
supply authority has earthed the neutral connector then a connection is made
from the active in the heater, through the water, through you, down to earth,
through the earth and up some transformer pole back to neutral. Good bye you
(possibly) but I would guess that the fish live to tell their future owner!
With an earthing probe, the current has 2 choices: you or the probe - the probe
made of copper and you made of, well flesh. Copper is a much better conductor,
so the probe gets it. You live.
DC is different: The electrons try to make a circuit from one side of the power
supply to the other. You CAN earth either side - usually the negative.
Therefore, current COULD pass through you to earth and then back to the
transformer, but 12V, 24V, 36V whatever would not be enough to do this. You
would need a very good conductivity path for a decent current.
As for current. I could quite happily run 5A at 240V AC through my body with
no possible chance of death. (I haven't by the way). We (Electrical people)
use current limiters of 5A in high voltage applications. The internal
resistance of you is tiny compared to these devices, so you have only a few
millivolts potential across you. Therefore, 5A WILL NOT NECESSARILY KILL YOU -
it's a combination of current and voltage drop. If 240V, 5A travels through
you to Earth without passing through any other resistance then yes, you get 5A,
240V AC across you and it will be serious.
Therefore, the chance of electric shock from DC voltages to Earth is low. A
word of warning though: you could get a shock if you provide an alternative
circuit back to the opposite polarity of the DC transformer in the power
supply. The most obvious would be by touching the two ends of say the
undergravel heating cable. It can be more complicated than this though.
Imagine the case where the positive heating cable has developed a fault and is
exposed to the metal lighting hood (which is not Earthed). Your power supply
negative rail is tied to Earth. You then lean over your tank, put one hand on
the hood and the other onto the metal tank stand (which is effectively Earth)-
once again, a shock.
In conclusion, always use Earth Leakage protection devices on AC equipment in
your tank - they measure the current flow between Earth and Neutral and switch
off if any is present. A DC system is safer in some ways, and generally takes
more than one fault to be present before you get a shock. A DC current
electric shock though is much worse for you than an AC one.
A none qualified Electrical tradesman should not wire things to the supply
mains - doing so will void you insurance policy - in Australia anyway, plus
it's illegal.
Just my comments,
Steve
Steven_R._Amor at ieaust_org.au