Electrical Mystery's:
I would suppose that the home electrical system is the single greatest source of mystery that the average home owner needs to contend with. This is probably true because other than the wires that carry the current it is invisible but deadly. Please bear with me as I try to give a step by step explanation of the wiring in the average US home. The transformer at the pole or other distribution point ( which is sometimes underground in newer developments) steps down or reduces the main line high voltage to the 240 Vac that comes into the dwelling. The secondary or house side windings of the transformer are center tapped so that there is a voltage of 120 ACV from each of the legs to the tap. The center-tap is tied to earth ground, first at the pole and also at the house site. It used to be very common to use the city water system as a source of ground, but that has become increasingly less common due to the introduction of plastic pipe and the discovery of corrosion that negated the effectiveness of such a grounding source. A heavy solid copper wire is now carried from the common or ground buss in the main electrical box to a copper plated ground rod that is driven into the ground. If we were to employ electrical distribution that did not include an earth ground we would soon discover that everything we touched shocked us because there would be a potential of about 62 VAC between the distribution system and ground. This condition is very common on sea going vessels as they use an above ground isolated system so that the hull of the vessel does not carry any current. Currents flowing through the steel of a ship will cause corrosion, deterioration and pitting of bearings, and produce magnetic fields which effects compasses, gyros and sets off magnetic mines. The 62 VAC is caused by the transformer action that exists between the ships wiring and the surrounding metal of cable shielding and the hull. If you will observe the electrical cable coming into your home from the pole you will notice that two of the wires are insulated while one us normally bare. The bare wire is the neutral or ground. This is called the neutral because there is zero potential between it and ground since it is in fact tired directly to earth ground. It should be noted here that the white wire is always tied to the "neutral" since of course white is a "Neutral Color." In the world of electronics black is ground because ground or dirt is black, but not so in the home, Black Is Hot. Water can be either an excellent insulator or an effective electrical conductor depending of the impurities such as salts that it carries. Pure distilled water will not conduct electrical currents and is circulated through the high voltage sections of some equipments such as high powered radio transmitters for cooling. We are all familiar with the static charge that can be generated by shuffling our feet across new carpet and reaching for the door knob or a lamp. This same effect can be created by water flowing through metal or plastic pipes that have no connection to earth ground. It will be noticed that the condition only exists when the water is flowing but quickly dissipates when the flow is stopped. Clamping a bare ground wire to the metal body of a faucet and attaching the other end to a ground rod will carry the offending currents to ground. It is sometimes possible to carry a current from a defective element in an electric hot water heater to the shower or tub faucets through plastic or ground isolated plumbing. I hope this over simplified explanation might be of some assistance in beginning to understand part of the home electrical service. Dale
Continuing Dale's explanation... This is known as Protective Multiple Earthing (PME). You make sure everything metal is grounded/earthed, so that if you were (as you have done) to touch to different (unconnected) pieces of metal you don't get a shock. In a bathroom all the taps (faucets/fawcets) should be earthed (grounded), as should a shower, electric towel rail, non-plastic bath (tub). I suppose even the toilet cistern should be earthed is it is metal, as should a metal urinal in the gents. You either daisy chain the earth from one tap to another or bring them all to one point. Then really you should take this back to the distribution board instead of relying on the water pipe - someone may come along at a later date and use a plastic connector. Elsewhere your central heating system should have a few earths on it, as should the incoming water supply pipe, and *possibly* the (natural) gas pipe - I believe the gas pipe should be earthed, but only the once - what you don't want to do is to make use of your gas pipe as a conductor in the event of a failure. Don't forget your kitchen sink, and any other taps in the house. (Hmm - that reminds me - I really should earth our outside tap, as I've used a plastic pipe half way along :-) Any external cables, whether they are encased in metal pipes or Steel-Wire-Armoured should be earthed. As should any 19" computer cabling cabinets. Don't rely on the Power Distribution Unit screwed into the back, as someone could easily remove this, or the plug; plus see below about thickness. And to make sure, add an earth connector for the doors and panels as well. So - earth everything!! I suppose you could include filing cabinets :-) Thickness of earth cable - needs to be nice and thick. You need a low enough resistance so that in the event of a fault a 30A fuse will blow quickly. If you have a bad (high resistance) earth path then it will take longer to blow, or worst case, won't blow at all. So, we don't rely on the earth conductor in the normal wiring, which may be 2.5mm^2 - instead we fit a 10mm^2 earth cable from the distribution board, and then possibly use 4mm^2 earth cable within the bathroom, between the taps. (Sorry I don't know what you wire gauges these would relate to in the US) Now, the none of the above applies if you have an Earth Leakage Circuit Breaker - in order for the ELCB to trip the earth fault current must flow back through the supply earth to the distribution board. Don't worry - most of you who think you have ELCBs probably have Residual Current Devices (RCDs) which measure the difference between supply and return - if there is 30mA or more difference (such as when someone/something is conducting electricity between supply and earth/ground) then it trips. Please note - continental European countries do not have a separate supply and return lead; neither of them are connected to earth. However, if you have two faulty appliances in the same building (or street?), one of which has an earth fault and the other a faulty earth you can end up with a hazardous situation. And...Over here in the UK (and Europe?) we use big yellow plugs and 110V transformers for site work. They are set up to be 55V either side of earth, such that the maximum shock you could receive is 55V if/when a cable or appliance becomes damaged. HtH, BJS PS - I wired our garage using two-core SWA cable, and using the armour as the earth conductor. To supplement this I drove a copper rod into the ground in the garage. I understand this to be 'marginally acceptable' according to the IEE wiring regs. PPS - I've got a question for you. Canada - Metric or Imperial??
He's right you know. Apart from the 0.1A bit, as this is 100mA - enough to kill you. I suppose in actual fact, as a shower circuit has up to a 45A fuse, and that there will be a couple of ohms in the circuit from the transformer, along the road, into your house, through your fuseboard, up to the shower, through the bath tub, back through the earth conductor(s), back to the transformer via the soil and a cable as well, this is really quite dangerous. Even if you put a 25mm^2 earth strap from your bath tub to your fuse board, you're still going to take a little while to blow that fuse. All this time you're in parallel to part of the earth circuit. So - always always use an RCD for your shower circuit, plus don't try to save money buying the thinnest cable you can get away with - its not worth it. I take it in the US you run 220V to your electric showers? The other problem you can get is fuse rupturing - I think they can actually superheat the vaporised fuse wire and get a conductive plasma - something weird and worrying. BJS