bigwow
Well-Known Member
I’ve just put a meter across the terminals of my galvanic isolator and it reads 7 Volts surly this can’t be right, or can it?
7V across terminals of galvanic isolator?
- Thread starter bigwow
- Start date
VicS
Well-Known Member
I think I'd want to investigate that further. Difficult to see how you'd get 7v ( sure its not 0.7) if it was OK.
Ac or Dc ?
Has it been subjected to a high fault current?
Ac or Dc ?
Has it been subjected to a high fault current?
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bigwow
Well-Known Member
I'm sure it was 7V and the meter was set to AC
markdj
Well-Known Member
You should be measuring DC, AC will be spurious. As the old chemist will tell you, putting 2 different metals in a saline solution creates a battery.
VicS
Well-Known Member
I'm trying to work out why you would get 7volts AC. You'd expect maybe to see a small DC voltage due to galvanic effects but even then not as much as 7 volts although the maximum will depend upon the design and spec of the GI.
Do you know its make or where we can look up any details.
Did you have any AC equipment in use at the time eg battery charger.
Switch it all off and see if you can identify an on-board source. Bad insulation or dampness in something perhaps.
Even so I dont believe that you should get 7 volts across it.
My first thoughts are that it could be defective but I would want to prove that before rushing out to buy a replacement. But how ?
Sure your onboard RCD is Ok . I am wondering why it has not tripped. 7 volts across the GI should indicate sufficient current flow to trip it. Measure the current flow if you have an AC ammeter but take care, you dont want to electrocute yourself ... wondering if you may have a fault on the AC sysytem and that the RCD is not tripping because the GI is faulty .... but there would be enough current going to earth via the anodes, propshaft and any other underwater bits and pieces connected to the earth to trip it I suppose.
Sorry thinking out loud trying to think of sensible things to try, tests to carry out etc
The maximum DC volts that would create would be around 1 volt. Rather more admittedly if they were magnesium and platinum but they are not much used in boat building AFAIK.
Do you know its make or where we can look up any details.
Did you have any AC equipment in use at the time eg battery charger.
Switch it all off and see if you can identify an on-board source. Bad insulation or dampness in something perhaps.
Even so I dont believe that you should get 7 volts across it.
My first thoughts are that it could be defective but I would want to prove that before rushing out to buy a replacement. But how ?
Sure your onboard RCD is Ok . I am wondering why it has not tripped. 7 volts across the GI should indicate sufficient current flow to trip it. Measure the current flow if you have an AC ammeter but take care, you dont want to electrocute yourself ... wondering if you may have a fault on the AC sysytem and that the RCD is not tripping because the GI is faulty .... but there would be enough current going to earth via the anodes, propshaft and any other underwater bits and pieces connected to the earth to trip it I suppose.
Sorry thinking out loud trying to think of sensible things to try, tests to carry out etc
Any pedantic old chemist will tell you that putting two different metals in a saline solution will create a CELL. It takes two or more cells connected together to make a battery!
The maximum DC volts that would create would be around 1 volt. Rather more admittedly if they were magnesium and platinum but they are not much used in boat building AFAIK.
William_H
Well-Known Member
7VAC across a GI
RFirstly it may be the meter finding funny things. Does it give any reading with either one meter lead attached? If you had a small capcitor like .1 microfarrad to connect across the meter terminals do you still get 7vAC. A digital volt meter has very high input resistance which means it can sniff out voltage that may not "really be there." On AC range the meter can react to strong radio signals or other AC induction even mains hum. Try a 10K ohm resistor or even a small 12v light bulb across the GI and meter. Of course if the light bulb lights you have a real AC fault and a dud GI.
If you really are getting 7V AC with any "substance" then the GI must be open circuit.
Try measuring ohms across the GI. (with it disconnected) there should be measured something in the few thousands of ohms.If it shows millions of ohms then it is suspect. Better if you have a diode test function on the meter. This should show 1.4v either way. Sorry this is all theoretical as I don't have a GI to try.
And as said try disconnecting mains supply if the voltage disappears then you have AC leakage and possibly a dud GI. good luck olewill
RFirstly it may be the meter finding funny things. Does it give any reading with either one meter lead attached? If you had a small capcitor like .1 microfarrad to connect across the meter terminals do you still get 7vAC. A digital volt meter has very high input resistance which means it can sniff out voltage that may not "really be there." On AC range the meter can react to strong radio signals or other AC induction even mains hum. Try a 10K ohm resistor or even a small 12v light bulb across the GI and meter. Of course if the light bulb lights you have a real AC fault and a dud GI.
If you really are getting 7V AC with any "substance" then the GI must be open circuit.
Try measuring ohms across the GI. (with it disconnected) there should be measured something in the few thousands of ohms.If it shows millions of ohms then it is suspect. Better if you have a diode test function on the meter. This should show 1.4v either way. Sorry this is all theoretical as I don't have a GI to try.
And as said try disconnecting mains supply if the voltage disappears then you have AC leakage and possibly a dud GI. good luck olewill
bigwow
Well-Known Member
I knocked off the RCD and the voltage went to zero. I think I'll get a sparks in to look at this as it's a bit over my head and rather worrying.
Thanks all
John
Thanks all
John
VicS
Well-Known Member
Its not surprising that turning off the power gets rid of the reading. I'd be very worried if it didn't!
Switch off all the individual items of mains powered equipment. ( including hidden things like water heaters) Put the RDC and the MCB on and check for the reading again.
If the reading reappears you have a problem with the shorepower installation itself.
If it does not then turn on mains equipment one thing at a time. That should identify anything defective.
Turn off the power
Bridge the terminals of the GI with a heavy wire ( as heavy as your shorepower cable earth conductor)
Turn the power back on, followed by all the mains equipment.
If the RCD and or the MBC trips it confirms that you have a fault on the shorepower installation or an appliance .. which ever trips it.
If it does not trip then all seems to be well with the shorepower installation and equipment .... but it indicates that the GI is probably open circuited.
I dont think you can check the GI with a multimeter on an Ohms range and i am not sure i would rely on a test on a diode check range either because I dont know what reading to expect.
If you get an electrician in make sure he knows what a GI is and has the proper facilities to test it.
Switch off all the individual items of mains powered equipment. ( including hidden things like water heaters) Put the RDC and the MCB on and check for the reading again.
If the reading reappears you have a problem with the shorepower installation itself.
If it does not then turn on mains equipment one thing at a time. That should identify anything defective.
Turn off the power
Bridge the terminals of the GI with a heavy wire ( as heavy as your shorepower cable earth conductor)
Turn the power back on, followed by all the mains equipment.
If the RCD and or the MBC trips it confirms that you have a fault on the shorepower installation or an appliance .. which ever trips it.
If it does not trip then all seems to be well with the shorepower installation and equipment .... but it indicates that the GI is probably open circuited.
I dont think you can check the GI with a multimeter on an Ohms range and i am not sure i would rely on a test on a diode check range either because I dont know what reading to expect.
If you get an electrician in make sure he knows what a GI is and has the proper facilities to test it.
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bigwow
Well-Known Member
Thanks VicS for that advise, I’ll try those tests when I go back to the boat tomorrow and update the thread.
rogerthebodger
Well-Known Member
To check a GI take a 12vdc battery and a bulb and make a circuit with the GI in series with the bulb. Measure the dc voltage across the GI should be 1.4 Vdc ish. Now reverse the GI connections and remeasure the GI, should also be 1.4 vdc ish. If not GI faulty.
Bulb should glow in both tests, if not also GI is faulty
Bulb should glow in both tests, if not also GI is faulty
bigwow
Well-Known Member
Just back from the boat. Over the terminals of the GI there is 7V. When the battery charger, Sterling 20amp, is switched on it goes to 37V!! what’s going on?
VicS
Well-Known Member
I cannot think that the GI is not faulty. I think it must be open circuited.
Disconnect it.
Connect it in series with a 12 volt bulb and a 12 volt battery. The bulb should light, perhaps not at full brilliance but at least it should light. Try it both ways round.
If it did not light I would be tempted to try the same with a mains voltage bulb and a mains power supply. But it is not something to do unless you can do it safely, I'd suggest on the workshop bench rather than in the bilges of the boat.
Definitely kaput if it did not power the mains bulb and almost certainly so if it did not light the 12 volt bulb.
Disconnect it.
Connect it in series with a 12 volt bulb and a 12 volt battery. The bulb should light, perhaps not at full brilliance but at least it should light. Try it both ways round.
If it did not light I would be tempted to try the same with a mains voltage bulb and a mains power supply. But it is not something to do unless you can do it safely, I'd suggest on the workshop bench rather than in the bilges of the boat.
Definitely kaput if it did not power the mains bulb and almost certainly so if it did not light the 12 volt bulb.
VicMallows
Well-Known Member
Along the same lines as OLEWIL I would be getting very suspicious about the (digital) meter readings. (some can act very strangely with a poor battery, for example, without giving any low-battery indication). This is an occasion when I would dig out the good old AVO or SIMPSON (old fashioned moving-coil multimeters).
Vic
Vic
bigwow
Well-Known Member
And the verdict on the battery charger? time to buy a new one perhaps
VicS
Well-Known Member
No not necessarily . It will be a switch mode type of device and therefore may well dump RFI into the earth connection. It may well be perfectly OK but will tend to turn the GI to conduct mode (if that were working )
If that is the case then it should be possible to put a capacitor in parallel with the GI to prevent this happening.
Cross the bridges one at a time
Sort out the GI first. Prove whether or not it is defective and replace it if it is.
If it is not defective I think we are all stumped.
If it is open circuited then William H's comments about a digital meter finding funny things may well be valid. If it is not then the meter should not find anything "funny". I'd not expect a reading in excess of about 1.4 volts.
Remember you do have the option of not bonding your shoerepower earth to the ships earth and Dc negative provided you have a "whole craft" RCD (and provided you do not have a metal hull) thereby doing away with the need for a GI.
skipp
New Member
Wow
If you can market it you will make a fortune, good luck with a solution.
If you can market it you will make a fortune, good luck with a solution.