4 Pole Breaker feed to Hazardous Areas

[davva]

I have read a passage in a text book that states "If the SWBD or MCC feeds equipment located in a hazardous area then a 4-pole circuit breaker should be used, as recommended in international standards".

There is no reference to the actually international standard where this "recommendation" stems from.

I have recently joined a company and their practice is to use a 3pole breaker with linked neutral bar at the distribution panel. The earth network usually being TN-S (separate neutral and protective earth). LV network 400V.

As the text book states it is only a recommendation I would like to get a feel for what the common practise is.

(I guess the source of the problem is that the neutral wire can rise to a potential above earth and create a spark hazard during maintenance?)

 

[alehman]

This should be resolvd by reviewing the applicable building codes. In the U.S., the National Electrical Code requires switched neutrals for circuits supplying fuel dispensers. To my knowlege, there is no such requirement for other types of hazardous areas in the NEC.

 

[waross]

The Canadian Electrical Code, 2006 Edition, requires:

Gasoline dispensing and service stations
20-010 Each circuit leading to or through a dispensing pump shall be provided with a switching means that will disconnect simultaneously from the source of supply all ungrounded conductors of the circuit.
Propane dispensing, container filling, and storage
20-040 Each circuit leading to or through a propane dispensing device or pump shall be provided with a switching means that will disconnect simultaneously from the source of supply all ungrounded conductors of the circuit.
Compressed Natural gas refuelling stations and compressor and storage facilities
20-070 Each circuit leading to or through a compressor or a dispensing device shall be provided with a switching means that will disconnect simultaneously from the source of supply all ungrounded conductors of the circuit.

No switched neutrals for hazardous locations in the Canadian code.
Under rule 10-106 (1) (b), and rule 10-210,
If a system incorporates a neutral, the neutral must be grounded.
An exception may be a circuit fed from an isolating transformer, but this would not generally apply to MCCs and SWBDs.
Furthermore, most equipment fed from an MCC does not incorporate a neutral connection.
Most equipment incorporating a neutral is single phase equipment, and a four pole breaker may be a little excessive for a single phase circuit.
respectfully

 

[mc5w]

Most U.S. fuel dispensing equipment ( not counting what is used to fill tanker trucks at bulk terminals ) runs on 120 volts single phase for the dispenser and 240 volts single phase for the pumps. U.S. rules require neutral switching for the 120 volt 2-wire circuits and 120/240 volt 3-wire circuits. In the case of bulk terminals the 120 volt or 24 volt control circuits would be running 480 volt motor controllers.

In my practice I use 24 volt isolating control circuits to drive the motor controllers or use a 120 volt controll circuit originating from the dispenser controller so that the 240 volt motor circuits do not need a neutral so that a cheaper 120/240 volt 2-pole breaker can be used for the motor circuit.

Most dispenser manufacturers ( exceptions such as Gasboy ) require all of the 120 volt circuits for the dispensers and motor controller coils to be on the same phase. One trick that we have used to get out of using expensive and hard to get neutral switching breakers is to run a 120 2-wire plus ground subfeed to a conventional single phase panelboard and put the neutral on the B-phase busbar and convert the neutral bar into an equipment ground bar. This allows us to use inexpensive and easy to get 2-pole 120/240 volt circuit breakers.

A variation that I have seen is to run single phase 3-wire plus ground power into a 3-phase panelboard and use the middle B-phase for the neutral. A plain 2-pole breaker substitues fora 2-wire neutral switching breaker and a 3-pole breaker for a 3-wire neutral switching breaker.

You should never ever use an ungrounded power system to supply power utilization load in hazardous areas because of the static electricity that can build up during rain storms. Ungrounded intrinsically safe circuits are O.K.

There was also at one time SquareD was peddling intrinsically safe push buttons and limit switches that used a fiber optic loop that was attached to a retooled photoelectric limit switch. This would also be a good technology for corrosive areas, wet areas, and so forth. I do not know if SquareD just simply discontinued the idea or if they sold it to somebody like they did with Telemecanique's pneumatic control relays and motor controllers that used a pnuematic piston in place of the coil.

As far as intrinsically safe goes, a lot of hot tubs use an air hose in between the button and the actual switch for the controls.

I have also figured out that the Grainger catalog has a UL listed fuel pump that runs on compressed air so that grounding only needs to be done against static electricity. Where you would want to do that is in a corrosive area that eats both galvanized steel and stainless steel.

Mike Cole