Like a tom cat ?
Why do we tie the neutral to earth?
- Thread starter eniacs
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Like a tom cat ?
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If you pull the centre of the winding to earth then view the two outputs on a scope, surely you'd see two sine waves 180degrees out with reference to earth?
like this:
http://www.beananimal.com/media/3246/split-phase-sine-wave.gif
I'm not really sure how that differs from two phases 180degrees apart?
like this:
http://www.beananimal.com/media/3246/split-phase-sine-wave.gif
I'm not really sure how that differs from two phases 180degrees apart?
There is only one winding (phase) so there can only be one sine wave. Nothing is displaced or leading or lagging. There is just a centre tap off the middle of the winding.
WRONG
If the supply is floating without any connection to ground ( or hull ) then there is no supply generated potential between any wire and ground.
There will be induced potentials and these could be very high voltage but ( hopefully ) very low energy meaning taking any current reduces the voltage to near zero. A fault in equipment could lift the supply to a dangerous level above ground (hull). A radar unit with a defective high voltage unit for its magnatron could create a high voltage potential with high energy capability between supply and ground.
All the ship born supplies I ever saw were referenced to the hull in some way. ( working for Staveley Smith Controls in 1969-70 )
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As the linked pic shows though, there are clearly two sine waves?
There are two measurements taken from different sides of one single sinusoidal formed voltage. The meaurements are referenced to the centre tap of the source and are therefore showing only half the voltage across the winding and are of differing polarity as the polarity of one of the measurements is reversed due to the probes being on different sides of the reference point
If the measurements were referenced to one end of the winding then one measurement would be zero, the other would be the full voltage
If the measurements were referenced to one end of the winding then one measurement would be zero, the other would be the full voltage
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Leaving ships out of it for now -
Looking at the supply network we could put forward a number of reasons for referencing it to earth. Taking just two:
1) It simplifies the problem of selecting the correct amount of insulation required for cables.
2) It facilitates the use of a simple reliable means automatic fault detection and isolation for the cable network.
For 1) Given that the general mass of earth is a conductor (hereafter called the 'earth') - any extensive supply network installed on or in the 'earth' will operate at some potential with reference to the 'earth'.
The potential to 'earth' at any point in the network would depend on the degree of coupling between the network and the 'earth' (generally capacitive coupling but some faults to earth may exist too). If we are to install cables that are insulated from 'earth' we would need to know this potential and this could be different values in different parts of the network.
If we reference our network to 'earth' (on LV and HV) at many points we eliminate this difference and we can produce cables that can be used anywhere on the network.
For 2) Given that the general mass of earth is a conductor - faults between the network and 'earth' could result in undetected current flows through the 'earth' and dangerous potentials between the network and 'earth'. A simple example would be if overhead lines came down - but many other types of fault are possible.
The above gives reasons why a supply network might be referenced to earth. This applies to all practical large networks included those that are not 'solidly earthed' such as IT systems. TN systems are generally solidly earthed at the LV level, IT systems are generally referenced to earth via a current limiter such as a resistor at the LV level. Some small IT systems are not earth referenced (small generators supplying the kebab van and the like).
These are some reasons why the supply network is earthed. Given that it is, this results in a potential risk to users of the network as it becomes possible to get a shock to earth - and this leads to the need to install protective measures such as earthing and bonding in electrical installations.
To the OP - I spent a number of years as an Electrical Officer in the Merchant Navy - I don't recognize the ship system you describe (I am of course familiar with IT systems on ships) - perhaps you could explain further.
Finally why earth the neutral (or more correctly the star point of the secondary winding of a delta / star LV transformer - well why not - that is the system we use - others are possible.
Looking at the supply network we could put forward a number of reasons for referencing it to earth. Taking just two:
1) It simplifies the problem of selecting the correct amount of insulation required for cables.
2) It facilitates the use of a simple reliable means automatic fault detection and isolation for the cable network.
For 1) Given that the general mass of earth is a conductor (hereafter called the 'earth') - any extensive supply network installed on or in the 'earth' will operate at some potential with reference to the 'earth'.
The potential to 'earth' at any point in the network would depend on the degree of coupling between the network and the 'earth' (generally capacitive coupling but some faults to earth may exist too). If we are to install cables that are insulated from 'earth' we would need to know this potential and this could be different values in different parts of the network.
If we reference our network to 'earth' (on LV and HV) at many points we eliminate this difference and we can produce cables that can be used anywhere on the network.
For 2) Given that the general mass of earth is a conductor - faults between the network and 'earth' could result in undetected current flows through the 'earth' and dangerous potentials between the network and 'earth'. A simple example would be if overhead lines came down - but many other types of fault are possible.
The above gives reasons why a supply network might be referenced to earth. This applies to all practical large networks included those that are not 'solidly earthed' such as IT systems. TN systems are generally solidly earthed at the LV level, IT systems are generally referenced to earth via a current limiter such as a resistor at the LV level. Some small IT systems are not earth referenced (small generators supplying the kebab van and the like).
These are some reasons why the supply network is earthed. Given that it is, this results in a potential risk to users of the network as it becomes possible to get a shock to earth - and this leads to the need to install protective measures such as earthing and bonding in electrical installations.
To the OP - I spent a number of years as an Electrical Officer in the Merchant Navy - I don't recognize the ship system you describe (I am of course familiar with IT systems on ships) - perhaps you could explain further.
Finally why earth the neutral (or more correctly the star point of the secondary winding of a delta / star LV transformer - well why not - that is the system we use - others are possible.
WRONG
Ships back then ran mostly on low voltage DC and used steam to power them. Things have changed since!
The HV side is referenced to earth with a 1 ohm resistor at the star point in the generators. But the LV side is floating. Your right if there was infinite resistance to ground then the voltage to ground could be any figure, but of course there is resistance to ground all be it in the MOhms.
I work for a popular british cruise line and have been there and measured it myself.
Also your right - in times of earth fault there can be high voltages generated - and odd voltages measured. I've measured 680V on a light fitting befor - it was a 220V fitting, but it was still working!!! We spend a considerable amount of time tracing and fixing earth faults!
How can there be any PD to ground if the supply is not tied to it anywhere?
I never realised that nobody in the USA has ever been killed by their 110V supply.
Measured what, between what?
Between what and what?
So a 220V lamp was quite happy with 680V across it?
1969 not 1869. Some old boats did have supplies that were not 230 AC and some had both AC and DC with DC mostly for the older types of motor with rheostat type speed controls.
Diesel engines some with fully automated controls. I recall sitting on the crank shaft to replace the temperature sensor for a big end bearing and hoping the thing didn't auto start.
It worked by catching the oil thrown off the bearing in a thermally insulated tray with the PT100 sensor in the tray.
We were investigating the ballasts on the circuit as they were only lasting a matter of days each time - it was 680VAC line to line measured with a fluke meter.
As i say the ground is the only reference point on the ship and its all connected together of course being a metal hull - so the cables from the transformers, all 100km's of them create a resistance to earth so this resistance pulls each line down to earth as close as posible (for a 220V system 110V each line...).
Of course it is posible to be killed on ship with electric - but if you touch a 220V line and there is no earth fault on that system then the line you touch is 110V PD, since the only way back to the other conductor is through the MOhms resistance of the cabling on the ship, the most you can feel is a tingle. If however theres a fault then you can get either 0V PD or 220V which certainly bites.
Hi Bernard;
Yes i was talking about 1969 too. Ships as recent as the Canberra and QE2 steam powered and DC drives!
Sitting on a crank shaft - i hope not inside the engine - certain death if it started up! We'd get sacked for entering a non isolated engine now. Which ship was that on by the way?
As i say the ground is the only reference point on the ship and its all connected together of course being a metal hull - so the cables from the transformers, all 100km's of them create a resistance to earth so this resistance pulls each line down to earth as close as posible (for a 220V system 110V each line...).
Of course it is posible to be killed on ship with electric - but if you touch a 220V line and there is no earth fault on that system then the line you touch is 110V PD, since the only way back to the other conductor is through the MOhms resistance of the cabling on the ship, the most you can feel is a tingle. If however theres a fault then you can get either 0V PD or 220V which certainly bites.
Hi Bernard;
Yes i was talking about 1969 too. Ships as recent as the Canberra and QE2 steam powered and DC drives!
Sitting on a crank shaft - i hope not inside the engine - certain death if it started up! We'd get sacked for entering a non isolated engine now. Which ship was that on by the way?
Yes inside the engine, locked of and the shaft pinned to prevent surges on the prop turning the engine, all crank case doors removed.
If I recall correctly, (it was 40 years ago) , it was either a very large tanker or bulk carrier that bought grain from the USA and took cars to the USA. We serviced anything with wires in it from radar to bilge pumps if there was not an existing UK service agent for the equipment. We also had contracts with many of the major suppliers to be their approved UK agents.
P
Paul_C
Stray capacitance in the wiring and connected appliances. It's A.C. remember.
That's one of the reasons why a floating system would become impractical for large-scale reticulation to homes. The larger the system gets, the higher the capacitance will be, and the greater current to earth that capacitance will support in the event of somebody touching a conductor. The isolation transformer for a shaver outlet is effective because it's a very small, controllable system. But to attempt to reduce shock risk by using a floating system on a large scale would not be practical due to capacitive effects, accumulative parallel leakage resistance to earth, etc.
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Ah DC ships I remember them well 
.
My first vessel was the SS Benloyal - built in the late 50's she had a steam turbine and a destroyer shaped hull. She managed 28 knots in sea trials but damaged her gear box and was later restricted to 21 knots service speed. She was probably the fastest merchant ship in the world for a short part of her existence.
DC supplied by diesel engines if I remember correctly (this was 1970). The alternators were LV as was common with all but the very largest vessels then and now.
In common with most ships she had earth lamps to detect first faults which implies that she would now be defined as an IT system. See appendix 9 of BS 7671 for examples of DC IT systems.
All the vessels I sailed on after that were LV AC supplied by diesel alternators and all had IT for engineering services. Hotel services were sometimes a form of TN-S derived via transformers with earthed secondaries. All had first fault detection on the IT systems - which means that they were earth (hull) referenced.
BTW the Canberra was powered by two turbo diesels driving electric propulsion and the QE2 was re-engined with diesels and a similar electric drive in 1986. The QE2 does generate at HV but as far as I know distribution is LV - mind you I have never been on it.
Now why is IT the preferred system for engineering (essential) services? - answers on a post card please.
. My first vessel was the SS Benloyal - built in the late 50's she had a steam turbine and a destroyer shaped hull. She managed 28 knots in sea trials but damaged her gear box and was later restricted to 21 knots service speed. She was probably the fastest merchant ship in the world for a short part of her existence.
DC supplied by diesel engines if I remember correctly (this was 1970). The alternators were LV as was common with all but the very largest vessels then and now.
In common with most ships she had earth lamps to detect first faults which implies that she would now be defined as an IT system. See appendix 9 of BS 7671 for examples of DC IT systems.
All the vessels I sailed on after that were LV AC supplied by diesel alternators and all had IT for engineering services. Hotel services were sometimes a form of TN-S derived via transformers with earthed secondaries. All had first fault detection on the IT systems - which means that they were earth (hull) referenced.
BTW the Canberra was powered by two turbo diesels driving electric propulsion and the QE2 was re-engined with diesels and a similar electric drive in 1986. The QE2 does generate at HV but as far as I know distribution is LV - mind you I have never been on it.
Now why is IT the preferred system for engineering (essential) services? - answers on a post card please
.Not him;
Yes QE2 was refitted with HV generation 9 huge turbo diesels and also fitted with the largest propulsion motors ever made. Canbera? It was a steam turbine driving the alternators - built in 1958 way to far back for medium speed diesels to be driving alternators!
What do you mean by IT system? I cant find an explantion of that one. On modern (ish) ships that i work on the engineering services are fed from the same method that hotel systems are. Except of course the high voltage Ac compressors, propulsion and thruster systems.
Yes QE2 was refitted with HV generation 9 huge turbo diesels and also fitted with the largest propulsion motors ever made. Canbera? It was a steam turbine driving the alternators - built in 1958 way to far back for medium speed diesels to be driving alternators!
What do you mean by IT system? I cant find an explantion of that one. On modern (ish) ships that i work on the engineering services are fed from the same method that hotel systems are. Except of course the high voltage Ac compressors, propulsion and thruster systems.
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