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Old 21-10-2013, 08:10   #76
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RCDs protect people by measuring the amount of current in the hot wire and the neutral wire. If they are unequal by a very small amount (30mA or less) the the device opens the hot and neutral wires. This works whether or not the boat ground is tied to earth or not. The idea is that any unequal current might be flowing in a person. It is highly unlikely a person could become shocked whilst keeping hot and neutral currents equal. That requires a specific intent on getting shocked such as sticking your fingers into a socket.

Ground wires came before the advent of RCDs (aka GFI). Ground wires provide a redundant path for current to flow in case the hot wire should accidentally come in contact with the case of a device. In that case a breaker should trip thus protecting an unsuspecting person who might touch the hot case of the device. Without the ground wire the case of the device would be hot but no current would flow until someone touches it.
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Old 21-10-2013, 08:39   #77
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Re: Isolating Transformers, The Earth wire connection conundrum

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That person may survive electrocution if an RCD is present in the wiring and if the RCD open in time.
CORRECTION. An RCD in this situation will not operate and not protect.
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Old 21-10-2013, 08:54   #78
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Re: Isolating Transformers, The Earth wire connection conundrum

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Hold on, there MUST be an isolation transformer installed to do this. The only way to get shocked is when your body completes the circuit between the two hots from the transformer. You are adding two failures, one for each hot. With that much wrong you would die because a moonrock woukd fall from the sky on top of you long before you get this fault.
Normally an isolating transformer used for personal safety consist of one isolating transformer and only one outlet then your moon rock theory may hold. One isolating transformer used to supply a complete electrical installation does not offer personal safety.
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Old 21-10-2013, 09:11   #79
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Re: Isolating Transformers, The Earth wire connection conundrum

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In that case a breaker should trip thus protecting an unsuspecting person who might touch the hot case of the device. Without the ground wire the case of the device would be hot but no current would flow until someone touches it.
To protect that person the breaker would require to open at less than 30 milliamps and in less than 10 Milliseconds which is unlikely.

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Since the function of the earth wire in the AC circuit is to provide a high reliability current path to ground
Unless the “ground” is electrically non conductive an earth wire is essential.
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Old 21-10-2013, 09:12   #80
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Normally an isolating transformer used for personal safety consist of one isolating transformer and only one outlet then your moon rock theory may hold. One isolating transformer used to supply a complete electrical installation does not offer personal safety.
An isolation transformer greatly reduces the chance of onboard electrocution. There is no need then to connect the AC safety ground to submerged objects and dock side earth. Thus ground faults are less likely to cause shock. No AC system is 100% safe unless there is no source of electricity. But an isolation transformer and RCD will make it about as safe as it can be.
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Old 21-10-2013, 09:17   #81
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CORRECTION. An RCD in this situation will not operate and not protect.
You were right the first time and the correction is wrong. If a person completes a circuit by touching a "hot" item such as a water pipe shorted accidentally to "hot" and the boat ground system a RCD will protect that person. The RCD has to be on the secondary of the isolation transformer but it will work.
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Old 21-10-2013, 11:34   #82
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Originally Posted by transmitterdan View Post
You were right the first time and the correction is wrong. If a person completes a circuit by touching a "hot" item such as a water pipe shorted accidentally to "hot" and the boat ground system a RCD will protect that person. The RCD has to be on the secondary of the isolation transformer but it will work.
In a true floating system, ie dual hots and an IT , RCDS on the secondary side provide very very little if any protection , in a floating system , since all currents including fault currents must return to the IT , the rcd will do little.

If a full wiring situation where the secondary side earth wire is connected to appliances cases and return to the one of the IT secondaries , in essence establishing a local " pseudo " hot and neutral system , RCDS can now detect imbalances and trip. However such wiring systems are in themselves trade offs.

In a full floating system RCDS do virtually nothing

Dave
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Old 21-10-2013, 11:36   #83
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Quote:
Originally Posted by transmitterdan View Post

An isolation transformer greatly reduces the chance of onboard electrocution. There is no need then to connect the AC safety ground to submerged objects and dock side earth. Thus ground faults are less likely to cause shock. No AC system is 100% safe unless there is no source of electricity. But an isolation transformer and RCD will make it about as safe as it can be.
Yes of fully floating , like Jedi , If however wired as per ABYC no additional shock protection is provided as the main point of an It per ABYC Is impressed current corrosion protection., not personal shock protection

Dave
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Old 21-10-2013, 21:05   #84
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Yes of fully floating , like Jedi , If however wired as per ABYC no additional shock protection is provided as the main point of an It per ABYC Is impressed current corrosion protection., not personal shock protection Dave
Thanks for holding the line Dave
I drove our coach from Santa Fe to Amarillo TX today.

What I can add is that if you want to protect against multiple failures stacking up to kill you, then it helps to have those rcd type outlets, I forgot how they are called. They will interrupt if any unbalance is detected at the outlet, like when a return through plumbing would occur.

A truly floating installation protects the whole boat, regardless of what Chala wrote IMHO. However, like Dave wrote above, you can't mix the ABYC recommendations into that because they don't understand the personal safety factor of IT's and kill that feature by grounding/polarizing the transformer output.
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Old 23-10-2013, 12:47   #85
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Re: Isolating Transformers, The Earth wire connection conundrum

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I can't bring up the incentive to discuss this silly repeat of having to ground a leg from a transformer or genset and live with a neutral. I will only say that this is the result of ignorance; this has been discussed to death too often now and many boats are 100% safe without a neutral and thus a floating system with two hot's.
The above is plain misconception.

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In a true floating system, ie dual hots
Dual hots! This term is ridicule because in the use of an isolation transformer for personal safety that consist of one only outlet none of the two wires are hot, each one of the two wires will allow no current to flow to earth.
In a Direct Current system the term Positive and Negative is normally used to identify each wire and each wire as an appropriate colour.
In an Alternating Current system, in a single-phase system the term Active or Line is normally used to identify one wire and Neutral, Middle wire or Common is normally used to identify the other wire. Each wire is identifiable by a colour that may vary depending on the country. Both wire are considered to be lives. Live (Alive): a term applied to an object when a difference of potential exists or would exist between it and earth under normal conditions of operation.


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The following threads and links are informative readings.
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Old 23-10-2013, 12:53   #86
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Chala,

I think you are confused. In a floating system as Jedi uses there is 110 or 220 V between the 2 "hots" but neither are referenced to "ground". So if either hot wire is shorted to ground nothing happens. Likewise if a person touches one of the hots and ground they will not be shocked. But an appliance plugged into the outlet will run perfectly. It is not ridiculous.
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Old 23-10-2013, 14:11   #87
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Re: Isolating Transformers, The Earth wire connection conundrum

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Likewise if a person touches one of the hots and ground they will not be shocked.
It is what I have written in the previous post.

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each one of the two wires will allow no current to flow to earth.
First some appliances have a tendency to leak to earth. Appliances that do not leak to earth are normally identified by a double Square and do not require earthing. Any other appliance that require earthing and is not earthed is a potential killer. Heater elements, fridge compressor, etc may leak to earth. Any appliance can be represented by a voltage ladder and any leaks may not be at the same step of the ladder which means that between two appliances there can be a lethal difference of potential. For the reasons of too many death the floating idea of Jedi was abandoned by most electrical supply authority many years ago and then RCD’s , originally called Earth Leakage Circuit Breaker, ELCB for Hellosailor, where introduced that required an earth to operate. I doubt that the system preconised will pass any electrical inspection, a RCD will never operate, such installation is just not safe.
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Old 23-10-2013, 14:33   #88
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a RCD will never operate, such installation is just not safe.
The RCD does not care if the neutral wire is grounded or not. It's function is not impaired by floating hot and neutral. All it cares about is that hot and neutral have equal current.
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Old 23-10-2013, 14:44   #89
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It is what I have written in the previous post.

First some appliances have a tendency to leak to earth. Appliances that do not leak to earth are normally identified by a double Square and do not require earthing. Any other appliance that require earthing and is not earthed is a potential killer. Heater elements, fridge compressor, etc may leak to earth. Any appliance can be represented by a voltage ladder and any leaks may not be at the same step of the ladder which means that between two appliances there can be a lethal difference of potential. For the reasons of too many death the floating idea of Jedi was abandoned by most electrical supply authority many years ago and then RCD’s , originally called Earth Leakage Circuit Breaker, ELCB for Hellosailor, where introduced that required an earth to operate. I doubt that the system preconised will pass any electrical inspection, a RCD will never operate, such installation is just not safe.
Chala, I'm afraid you haven't the faintest idea of what you are talking about , an isolating transformer with fully floating outputs has no earth connection, ie earth is not in the circuit. Hence there can be NO earth leakage from an appliance.

Your concept of voltage ladders is faulty as well and your concept ( common mode voltage ) is only really an issue where multiple transformers are being used ( like in many pc connected together etc ) in a whole boat IT transformer this issue doesn't arise

Your understanding of RCDs is also faulty , RCD trip when the current passing in one live wire is not the same as the other. It doesn't matter where the missing current goes. RCDS Offer little protection in fully floating IT systems , primarily because with no fault circuit to ground , it's unlikely the conditions will exist to trip it, this doesn't mean the system isn't safe , it actually means the system is safer then an RCD protected system.


Note on nomenclature : the outputs of a transformer ( not centre tapped) can both be considered live , or hot , I know hot has a particular connotation in the USA, but elsewhere it used interchangeably with " live" , as in "this circuit is hot" . Neutral on the other hand has a specific designation , often misused. Because some countries have floating " neutrals" which aren't neutrals at all.

Dave
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Old 23-10-2013, 16:37   #90
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Re: Isolating Transformers, The Earth wire connection conundrum

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Chala, I'm afraid you haven't the faintest idea of what you are talking about , an isolating transformer with fully floating outputs has no earth connection, ie earth is not in the circuit. Hence there can be NO earth leakage from an appliance.
Dave,
Are you saying that an appliance can not leak current to ground simply because it's power source is the floating secondary winding of an isolation transformer?

In the case of a refrigerator that has a steel shell that happens to touch some metal part of the boat, if the compressor motor leaks some current to the metal shell, would that not be an example of current leaking to ground?

If a hot plate were also in use & the other power leg leaked current in that appliance, would that not create a hazard for a person holding a pan to fry some eggs while touching a metal surface in the boat, or reaching to grab another egg out of the refrigerator?

My concern would be that just because a secondary winding was not grounded at the transformer, does not mean that an appliance down stream can not create a ground loop there.
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