Earthing a portable generator
- Thread starter Forest Girl
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prv
Well-Known Member
It's a very long time since I mugged up on this stuff to do a safe job of wiring my shed, and I've mostly forgotten it again. So probably best if I just link to http://wiki.diyfaq.org.uk/index.php?title=Earthing_Types
Pete
davidpbo
Well-Known Member
TT = Terra (Earth) Terra i.e there is a connection to the mass of the planet, via a spike or similar at source and destination. Neutral is taken to earth at the transformer. There is no protective conductor connecting the supply to the installation. Earthing within the installation is provided locally by earth rod or similar. Often used in rural properties I believe.
TN-S Terra Neutral Separated. Neutral and protective (Earth) conductors separated. Supply N is earthed at the transformer.
TN-C-S Terra Neutral combined separated. Transformer N taken to earth, protective conductor and neutral combined in the distribution system and separated in the installation. The most common method in new installations I believe.
If I am seriously sorting something on the work bench at 240V, e.g. something with a switch mode power supply, I use a mains isolating transformer 240/240 with no earth connection on the secondary. I can touch live and chassis or neutral and chassis of the equipment and not get a shock, if an Rcd was in circuit I doubt it would trip. If I touch Live and neutral in the equipment I will still get a shock and would (I believe) if there was an RCD in circuit and I doubt it would trip.
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rogerthebodger
Well-Known Member
I ask again how does that imbalance get back to the supply and it just cannot disappear into thin air or the earth. It must go somewhere.
Don't know what kind of "plug-in" RCD you use but again the imbalanced current must get back to the generator circuit bypassing the RCD.
It's based on a fundamental law of electrical current flow.
In this case Davidpbo is creating what you say would trip the RCD but it doesn't
If it does in your case you have a setup different to what you think you have.
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VicS
Well-Known Member
An rcd will trip if there is a current flow to earth through you.
It wont trip if you touch a live conductor while standing on a dry non-conducting floor or wearing your industrial safety shoes because there is no effective path to earth.
If however you are standing in a typical rain soaked English boat yard wearing non protecting foot wear it will trip because it has detected an imbalance due to the current flowing to earth though you. Current that would electrocute you but for the RCD
Touching live and neutral simultaneously, while you are insulated from earth, can electrocute you without tripping the RCD.... because there is no imbalance between the current in the live and neutral conductors.
rogerthebodger
Well-Known Member
Yes that is true only if there is an earth spike connecting the earth we walk on to the neutral of the substation.
If there is no earth spike connected to the substation neutral there can be in effect not return path back to the substation like if you are standing on an insulated pad, a dry non-conducting floor or wearing your industrial safety shoes.
The electrical path through the earth you stand on is not always a perfect electrical path so we fit an "earth wire" that can be made and tested to be a good electrical current path. That is way we should IMHO have one but in lots of places around the world it is not so good as the UK and most commonwealth countries like South Africa which had a legal requirement for a RCD in domestic installations before the UK and before I came 32 years ago.
The imbalance every one talks about must find its way back to the substation "neutral" some way, just flowing to the earth we walk on is not enough.
Consider the substation on a boat as an isolation transformer earth wire/neutral/ earth we walk on all or sea water, all connected together.
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Richard10002
Well-Known Member
I think you misunderstand me... I have earth and neutral connected in the lead that I use to connect my generator to the shore power inlet. I also stressed that you should not use this lead to connect to the shore supply.
The OP asked about earthing the generator... By default, one might assume that there is no connection to a shore supply.
The very important thing is not to use the lead with earth and neutral connected to connect the boat to a shore supply!!!
VicS
Well-Known Member
Before making up a lead with neutral and earth connected I would check the generator to see if this is already done internally.
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Richard10002
Well-Known Member
I have one of those 3 pin plug type circuit testers, (various combinations of 3 lights show the fault). Mine showed no earth with generator connected, earth OK on shore supply. After much deliberation and delay, (possibly a year or so), I connected the earth and neutral in the genny lead, and the tester showed earth OK.
As I said, I don't understand these things, but after a bit of deduction, it seemed to make sense.
I agree that you should test for an earth fault before doing anything.
VicS
Well-Known Member
I have checked the wiring diagram and see that the samll hondas are not in fact connected internally
Nothing wrong with your powers of deduction.
Graham_Wright
Well-Known Member
Interestingly, I have noted that, when working on a mains circuit with the live not connected, an earth neutral short will trip the breaker. I guess the neutral must float up sufficiently for there to be a pd between it and earth due to other circuits.
VicS
Well-Known Member
yes I have noticed this too . In fact i think it was a ( deliberate ) earth to neutral short which caused one of my plug in RCDs to fail. Tripped , but would never trip on the test button again
Laundryman
Well-Known Member
Okay, I'm just about to make myself a lead, specifically for the generator to boat supply. At the plug which goes into the generator I have to connect both the neutral and earth together. To which pin please, the neutral pin or the earth pin? I presume that I needn't make any change to the plug on the other end which connects to the boat. I've never thought of myself as a thick , but I must be! Thanks. Alan
VicS
Well-Known Member
You must use the "neutral" pin.
But if you connect both wires to just the neutral pin the generator chassis and earth terminal will be left floating.
If you connect to both neutral and earth in the normal way and also link them in the plug the generator chassis and earth terminal will be connected to the boats internal "earth" wiring. I'd do this.
The above assumes that your generator is wired internally the same as a small portable Honda. You should refer to the wiring diagram of your own generator.
leave the boat end normal. ( although if more convenient you could make the link at that end)
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rogerthebodger
Well-Known Member
D
Deleted member 36384
Guest
I have a 5kVA 240V generator with an external earth stud (not connected to the ground or a spike) and it gets plugged into the yacht's shore power via a home made crossover lead 3 x Pin domestic to 3 Pin caravan style plug round pin wired up normally i.e. L, N, E. The generator is a mass market type that you see sold at Machine Mart, mines is a bit older than the current models, so nothing fancy. Can you confirm my understanding please: -
1. At the moment the RCD in my yacht mains ring circuit (professionally installed) will not work because it can't sense the required voltage difference if there is a fault. This is a dangerous condition because I could get an electric shock and would not be protected by the RCD.
2. For the RCD to work, I need to connect the earth and neutral pins with a short length of insulated wire in one of the plugs of my home made cross over cable.
If I am wrong please just say so and there is no need to explain it all again in the thread. I will then seek professional help.
Thanks,
BlowingOldBoots
1. At the moment the RCD in my yacht mains ring circuit (professionally installed) will not work because it can't sense the required voltage difference if there is a fault. This is a dangerous condition because I could get an electric shock and would not be protected by the RCD.
2. For the RCD to work, I need to connect the earth and neutral pins with a short length of insulated wire in one of the plugs of my home made cross over cable.
If I am wrong please just say so and there is no need to explain it all again in the thread. I will then seek professional help.
Thanks,
BlowingOldBoots
VicS
Well-Known Member
It detects a current imbalance not a voltage difference
The RCD requires the neutral to be connected to the earth to be effective ( unless its connected its not a neutral at all )
Lots could be said but you request that it isn't
GrahamM376
Well-Known Member
As a matter of interest, how do you know the RCD isn't working? If it doesn't trip when you press the test button, that would indicate a faulty RCD as it doesn't need an earth to work.
http://www.electriciansforums.co.uk...gulations/38819-yet-another-rcd-question.html
D
Deleted member 36384
Guest
It does work, I test it every time I power up the genny but I have no idea what the test button does: does it replicate a fault, or just verifies that trigger will release the switch. I don't need or want to know. I was trying to establish if my understanding was enough to connect the neutral and earth together so that if there was a current imbalance the RCD would trip.
I'll ask an electrician at the yard.
captainboo
Well-Known Member
The test button allows the correct operation of the device to be verified by passing a small current between the live and neutral such that the current only flows through one side of the sense coil. This simulates a fault by creating an imbalance in the sense coil. This is why no earth connection is required for test purposes. If the RCD does not trip when this button is pressed then the device must be replaced.
