should be an approved double-pole type (to switch the phase and neutral conductors of a single-phase mains supply).

Europe and the UK require double pole switching for many mains disconnect applications.

Can any US Electrical Engineer cast light on the US regulations please?

 

All hots must be disconnected simaltaneously, so it depends on the number of phases. In a single phase, a single pole is adequate

 

We don't switch the neutral over here, just the live conductor(s)

 

We don't switch the neutral over here, just the live conductor(s)

Thanks for the reply.

Does this include the electricity company supply switch for a house incoming supply?

Also, what about emergency stop switches to industrial machinery?

 

Also, what about emergency stop switches to industrial machinery?

Most all industrial machines use "contactors", magnetic switches to disconnect the Hot side of the mains. This keeps the machines from restarting on their own when power is restored after a outage, and to isolate when shut down on purpose.

 

further to the contactor arrangement, E stops interrupt the control power to the contactor, preventing it from being pulled in.

In more elaborate arrangements, the control power (used to pull in the power contactors) is itself switched with a relay, which in turn is interrupted by the E stop. This relay is sometimes to referred to as the 'Master Control Relay' or MCR.

Lockouts and/or Isolation switches are different again, in that they disconnect the mains themselves. Contactors are not used for isolation. Semiconductor devices are not suitable as isolation switches.

 

Also each machine, ac unit, etc must have a "local disconnect" within sight of the machine.

 

The typical US house has single-phase 240 VAC coming in on three wires from a center-tapped step-down transformer. The center tap is connected to an earth-potential ground (provided by one or two grounding rods driven into the ground near the power entrance) at one place in the circuit breaker panel. The center tap is called the neutral inside the house. The house's wiring then has a number of 120 VAC circuits, wired from one hot side of the transformer to the neutral; the electrician tries to balance the number of these 120 VAC circuits so that the currents are roughly equal in the two hot legs (the neutral current is the difference of the two hot leg currents). These 120 VAC circuits use one single pole circuit breaker. The wiring run for 120 VAC circuits in the house consist of three wires: one hot wire, one neutral, and one grounded conductor. Excluding IR drops in the neutral wires (which should only be on the order of a volt or so), the only non-ground potential conductor in a properly-wired 120 VAC circuit is the hot conductor.

Higher-power appliances (e.g., dryers, stoves, heat pumps, air conditioning, water heaters, well pumps, and shop equipment) are run on the 240 VAC power. These all require two pole circuit breakers and disconnects. The power coming into the house also requires a two pole disconnect. The 240 VAC circuits' wiring consist of two hot wires and one ground wire.

 

On 240v items, like air conditioning compressors, L1 and L2 are broken.

Keep in mind that in the USA, power is split-phase; residences get two 120VAC lines that are 180° out of phase, and a Neutral line; basically the power transformer secondary is center tapped.

The center tap is called Neutral, and is connected to earth ground at the electrical service panel via a solid copper AWG-4 (5.2mm or larger) conductor connected to two 8-1/2 foot (2.6 meter) long copper-clad grounding rods placed at least 6 feet (1.83) meters apart.

 

I haven't bought the 2011 NFPA-70 yet and haven't had to reference the current issue as to ground rods, but in the 2005 edition a single ground rod 8' in length was considered sufficient if it tested as sufficient.

In the buildings we used to be in I only needed one rod on three of them but had to use a second on one that had nothing but parking lot around it; the ground underneath was apparently pretty dry. Stupid buildings had nothing when I started working there.

[Note to myself: Add copy of the 2011 code book to my "to buy" list, NFPA 1 and 101 are still current.]

Yea, it's not cheap to buy a new copy of NFPA-70, but once a new issue appears you can often find a copy of the previous edition pretty cheap on Amazon, eBay, a local electrical or book store - perhaps even a contractor may be willing to give you one of their superceded ones. It has to be some of the best, most confusing reading you'll ever have on your bookshelf but "Once you go Red you'll rarely go dead."

Cheap shot at a joke but seriously - it's worth reading through one of these as you might actually learn something.

 

You can view the latest NEC (NFPA70) online at their site for free... www.nfpa.org
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