Reverse polarity. Dangerous?

AndrewB

AndrewB

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Got back to the marina from a sail yesterday to find all mains power sockets on my nearest post were taken. My cable only reaches one other post with an empty socket, and I know that one is wired wrongly. Sure enough it triggered the onboard reverse polarity alarm. But other boats use it, so I just went ahead. Is that dangerous? Is there equipment that can be damaged by reverse polarity?

(It's been wrong for over a year. I've told the marina staff there is a problem, but they don't take it seriously - or, I rather suspect, they are too stupid to understand and pass the message to their electrician. Of course, I could have rewired my plug, but then I would only have had to rewire it back normally today when I could get back on my usual post).
 
Alternating current reverses polarity 50 times a second so nothing will be damaged by it. However swapping live and neutral means that if you have equipment with a single pole switch in the live side it can be switched off but actually still live inside. This is potentially dangerous if you then come into contact with an internal part of the equipment. Not very likely but still not a good idea. A lot of continental plugs can be inserted either way and they seem to get by even though by UK standards this seems wrong.
 
Yes, it is potentially dangerous. Most likely your fuse only breaks the hot wire. If reversed polarity in case of over current the fuse will break the neutral. And if you over current is caused by a short between hot and ground to break the Neutral will not solve the problem, so better be very careful!
 
Yes it is potentially dangerous. In UK neutral is essentially earth and the live alternates. Short neutral to earth and not a lot happens (although a breaker may trip). Short live to earth and the fuse will blow. Swap live and neutral and the fuse is no longer protecting anything. A trivial fault with the appliance could have serious consequences. If you do have a RCD in the circuit then it is less serious as that would trip in either configuration.

If the marina won't sort it out then make yourself a polarity reverser.
 
For the future, get a free-standing plug and a socket, and a small length of cable, and make up a reverse polarity corrector. Label it prominently that it is only to be used for that purpose. For the marina that won't fix it, bandy around a few choice phrases like Health & Safety, Duty of Care etc and that you have made a written note of you having informed them.
 
AndrewB said:
....(It's been wrong for over a year. I've told the marina staff there is a problem, but they don't take it seriously - or, I rather suspect, they are too stupid to understand and pass the message to their electrician. Of course, I could have rewired my plug, but then I would only have had to rewire it back normally today when I could get back on my usual post).
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Have you done this in writing/e-mail? Or just verbally to a junior rank who doesn't understand and maybe doesn't care?
 
andsarkit said:
Alternating current reverses polarity 50 times a second so nothing will be damaged by it. However swapping live and neutral means that if you have equipment with a single pole switch in the live side it can be switched off but actually still live inside. This is potentially dangerous if you then come into contact with an internal part of the equipment. Not very likely but still not a good idea. A lot of continental plugs can be inserted either way and they seem to get by even though by UK standards this seems wrong.
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Most European standards require a double pole breaker hence you can go both ways around!
 
andsarkit said:
Alternating current reverses polarity 50 times a second so nothing will be damaged by it.
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If the current keeps reversing polarity, how does my polarity alarm recognise the difference?

As I have a double-pole isolator and an RCD fitted aboard, it does sound from what is being said that it should not be dangerous.
 
AndrewB said:
If the current keeps reversing polarity, how does my polarity alarm recognise the difference?

As I have a double-pole isolator and an RCD fitted aboard, it does sound from what is being said that it should not be dangerous.
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Yes that should mitigate it.

Ultimate mitigation comes from an isolation transformer of course. With galvanic protection advantages. They are pretty heavy though!
 
AndrewB said:
If the current keeps reversing polarity, how does my polarity alarm recognise the difference?

As I have a double-pole isolator and an RCD fitted aboard, it does sound from what is being said that it should not be dangerous.
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The other risk of course is that many devices have a single pole power switch in the live feed. With the switch off nothing on the device is live so it is safe to work on.

With the polarity reversed the switch will still work, but when switched off a lot of the device will still be live - i.e. dangerous to work on. A functioning RCD will dramatically reduce the risk to life but I wouldn't knowingly operate with reversed polarity when the fix is so simple (a polarity reversing connector)

To answer the question, in UK Neutral should be at the same potential as Earth and the Live alternates between positive and negative relative to N/E. With reversed polarity the Neutral will be alternating and it is easy to detect because there will be a potential between N and E. Something as simple as a Neon between N and E will detect reversed polarity
 
andsarkit said:
A lot of continental plugs can be inserted either way and they seem to get by even though by UK standards this seems wrong.
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Their wiring system getting towards the final circuit is I think different. Stick a voltmeter between L1 and L2 you will get 220V, between L1 or L2 and earth and you will get 110V if I recall correctly. Hence they use double pole breakers.

Try that in the UK you will get cc. 235V between live and neutral and between live and earth. Between N & E the voltage should be virtually non existent in most instances.

Hence why polarity is important in the UK where a lot of single pole breakers are used.

A single pole breaker or fuse or switch must be in the live leg of the circuit to do its job in the safest way.
 
The system described above in post #13 is of a two-phase system with two live wires. A similar system is used in the UK for site tools operating at eg 120 volts, where the maximum voltage on either of the live wires is only 60 V RMS, which is safer than the possibility of contacting a 120 volt live wire on a 120 volt single phase system and a whole lot safer than doing the same on our 240 V RMS UK mains. As a matter of interest, the actual voltage of the live wire swings between + and - 340 volts as compared to earth.
 
Personally I would never work on a connected device or on-board wiring without completely disconnecting the mains supply. So reverse polarity is just a niggle with the red warning light on the switch panel!
 
Hoolie said:
Personally I would never work on a connected device or on-board wiring without completely disconnecting the mains supply. So reverse polarity is just a niggle with the red warning light on the switch panel!
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Which is of course good practice, but people make mistakes, forget things etc. Would I be too bothered on my own boat? Probably not for a limited stay.
Everything I was using was Class II anyway.
 
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AndrewB said:
they don't take it seriously - or, I rather suspect, they are too stupid to understand and pass the message to their electrician.
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How delightfully patronising of you. I'm sure the quality of service you receive is in no way related to this.
 
Hoolie said:
Personally I would never work on a connected device or on-board wiring without completely disconnecting the mains supply. So reverse polarity is just a niggle with the red warning light on the switch panel!
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My thoughts as well. The boatyard we use connects with 2 pin Euro plugs so 50/50 chance of reversal but everything works fine. I never work on mains circuit without isolating via double pole switch immediately after shore socket.
 
Not being funny, but nobody with any competence would ever work on something that was still connected or not locked off, and hadn't been checked by themselves, before opening it up, anymore than they would check a gas system with a match.
As stated by others, the provider should be 'really' made aware of this potentially fatal break with good practice.
 
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