Earth Fault Relay in HV System

The earth fault relay measures the vector sum of the three phase currents (or three phase current plus neutral for wye connected). This is done using three current transformers, and if there is no fault, the sum should be zero. If the sum is not zero, the earth fault relay triggers.

http://circuitglobe.com/restricted-earth-fault-protection.html

Def: A neutral conductor is an intentional current carrying conductor. A ground, or earth, is an unintentional carrying conductor.

In a single phase system the ground fault monitors the line and neutral currents. They should be equal, and if the line current is higher, it must be going through another path than the neutral, typically the ground. This is considered a circuit fault, and the device disconnects the line from the circuit.

And if he needs to ask on this forum it's more likely to be about 1- than 3-phase.

Codemaster...

I see you are from the UK...

Is the 'Ring Main' system still in use there? I assume its benefit is less voltage drop and continued power in the event of an open in the circuit, but unfortunately that open circuit might not be evident by the user. Is the complication of installing it worth the benefits?

I assume ring main is still used in UK, but I'm not an electrician so not 100% sure. I see what you mean about an open circuit not giving obvious symptoms, but if it happened, there would have to be a heavy load to cause a problem.

Also in theory the method is to bare a few inches of the cable, expose an inch or so of conductor, and bend it a tight u-shape to insert in the terminal, so if it falls out it doesn't interrupt the ring. Whether sparkies bother with that in practice I wouldn't know.

Depending on house size, ring likely to be cheaper than a separate feed to every outlet. Whether it's cheaper than a tree-type layout might be a close call. Probably depends on the individual property.

<...assume...> The assumption is incorrect.

The method was actually adopted for new installations following the 1939-45 conflict as a way of wiring up a house using rather less copper than radial wiring would otherwise; copper was in short supply and of high value at the time. The principle depends upon having a fuse in the plug that connects the final equipment. Hence the "13 amp plug" that is commonplace here.

There is a good page in Wikipedia on appliance plug types across the globe.

OK thanks for the correction, but what is the current system? My first house was built in 1973, my present one in 1963, and I'm fairly sure they both use the ring system. Minimising the quantity of copper sounds like a good idea whether there's a war on or not!

I haven't time to check Wiki at the moment, but sounds like it deals with plugs and sockets, not circuits.

I found Wiki data on plug types, but that's all it was, not wiring systems.

I also found UK house wiring which appears to confirm my original comment. If you don't agree that (mainly) ring main is the current system, what system do you think is used these days, and do you have supporting information? I'm always ready to learn!

Probably more than you want to know about U.S. wiring!

Here in the U.S., separate circuits are not usually run for each outlet. Several outlets are connected to the same circuit, looping in and out, but ending at an outlet, not returning as in a ring scheme.

Depending on the area and use, the applicable codes will specify the number of outlets required, and placement is often based on wall length and use. Kitchens require more dedicated outlets because of potential cooking appliance loads. Laundry, water heater, heating and air conditioning are also separate circuits. Lighting is connected separately from the outlet circuits and often rooms are grouped together.

Ground fault protection is required for many areas, like kitchen, bath, basements and outdoor. The use of ground fault outlets is more common than ground fault breakers because they also offer protection for downstream outlets without dedicated wiring. GFI breakers tend to false trip with exterior wiring if the wire gets damp but no actual fault occurs in the protected receptacle. Arc fault protection is also required for many sleeping area outlets.

The most common outlet is the duplex, with two outlets on a common frame. They can be used as a pair on the same circuit, or by breaking off a link, can be used for two circuits. For example, one half can always be powered and the other half switched. This gives the ability to use a table lamp with a wall switch instead of, or in addition to, a ceiling fixture. Ceiling fans are usually wired with a 3 wire cable to allow wall control of the fan and light, if installed.

With the device mounting frame size standardized, a mix of outlets, switches, dimmers, pilot lights and timers can all be installed in a multiple box. The newer 'decorator' style devices, also available in any device type, plus other configurations all use the same frame size, but the face plate openings are all the same design. Mixing devices has become easier since only the device count, not the type, needs to be considered. With the increase of audio and communications wiring, divided enclosures are now being used to allow line and low voltage circuits to share the same face plate with required isolation inside the enclosure.

The most popular residential wiring is Non Metallic Cable available in 2,3 and 4 wires, with ground, in various gauges. Years ago the color of the jacket was meaningless d but now there has been an evolving standard jacket colors. White 15A, Yellow, 20A, Orange 30A and Black for 44 or 60 A. and gray for exterior use. I guess this because many inspectors have poor vision

The NEC (National Electrical Code) is an industry standard that is adopted either in full or with changes by the local authority having jurisdiction. The standard residential mains supply is 120/240V at 200+ amps, with 20 or more individual circuits. 120 volt, 15 and 20 amp circuits are the most common, and larger ampacities are used for the 240 volt circuits for larger loads.

Rarely are outlets controlled by switches, as is common in Europe, except for special use and the split wired duplex mentioned above. Fused plugs are used only on festive lighting, like Christmas lights, because of their small gauge wire, and rarely elsewhere.

Certainly, the wide European use of 240V requires less robust hardware and wire because of the lower current for an equivalent wattage. I work on industrial equipment, and marvel at the size, compactness and reliability of the 'foreign' electrical components in comparison to what is considered to be 'standard' US devices.

Cheers