Fuel Pump Disconnect

[wirestu]

We are working at a site that has a small electric gasoline filling station. The pump/controller requires three circuits (1 for each pump and 1 for the electronics). These circuits are fed from breakers that open the neutral and the feed. Locally, near the pump they wish to add disconnects. Our question is do these disconnects have to open the neutral also? What is the reason behind this. Also there is an emergency stop relay activated by a pushbutton. Does this relay also have to open the neutrals.

Thanks for the replies,

Stu

 

[rbalex]

Re: Fuel Pump Disconnect

I believe 514.11(A) answers your question.

 

[stew]

Re: Fuel Pump Disconnect

the code reference gives you the direction to do what it says but does not actually give you a reason for this. When I took my haz loc class I asked the same exact question as you have asked. My instructor pointed out that the reason was that in doing so the even the remotest chance that an arc could occur is eliminated.

 

[rbalex]

Re: Fuel Pump Disconnect

The NEC (most codes, in fact) rarely explain the underlying reasons for specific rules. Occasionally a FPN may reveal it. ROPs, ROCs or their equivalents will usually explain where they came from, but some are so old that the research is difficult.

I was trying to research my old documents for the specifics. The rule is much older than what I have immediately available. In any case, your instructor was fundamentally correct. The NEC handbook gives a more complete answer:

The intent is that no energized conductors are in the dispenser vicinity during maintenance or alteration. Considering possible accidental reversal of the polarities of conductors at panelboards, the grounded conductor must be able to be switched to the open or off position.

 

[don_resqcapt19]

Re: Fuel Pump Disconnect

Even if the polarity isn't switched, you can still get an arc if the grounded conductor is still connected to the neutral bus at the panel and you cause it to touch a grounded surface at the dispenser.
Don

 

[rbalex]

Re: Fuel Pump Disconnect

This is true. A sufficiently long current carrying grounded conductor can develop a voltage potential to ground and cause an arc. But that would be true whether the disconnect opened both conductors or not. If the rest of the installation is proper there is no more concern for an arc than there would be in an ordinary location. Arcing devices are commonly installed in Division 1 locations.

Pole reversal during maintenance was the original reason and it was based on an actual series of events. It goes back further than my time in the business although I recall reading some of the documents. As I recall, the worker involved had opened the disconnect before working on the dispenser. He then opened an explosion proof enclosure inside the dispenser shell where hot and neutral had been reversed at the panel. There was still sufficient ignitable material to cause a fire when the arc occurred.

In Division 1 locations there is a general prohibition for multi-wire branch circuits unless all conductors including the grounded conductor are simultaneously interrupted. I must confess I haven?t looked at the proposal that originated this, but I suspect it was based on a combination of 210.4(C) Ex2 and 514.11. I also suspect it wasn?t based on an actual event.

 

[don_resqcapt19]

Re: Fuel Pump Disconnect

rbalex,

A sufficiently long current carrying grounded conductor can develop a voltage potential to ground and cause an arc. But that would be true whether the disconnect opened both conductors or not.

That is not what I am talking about. If the neutral is connected to the bus and then you make an accidental connection to a grounded or bonded object, you have created a parallel path for grounded conductor current and you will produce a small arc. This is especially true where the circuit does not originate at the service panel. When you open the connection to the grounded bus at the panel, you eliminate this possible parallel path.
Don

 

[stew]

Re: Fuel Pump Disconnect

Exactly my point as well don. I used to work for Amerigas doing installs on propane pumps and maintanence on existing units and plant pumping equipment .Thats why I took the haz loc course. I learned a tremendous amount about xp that I was not at that time aware of. Had a super Instrutor MR Joe Andre who is now with UL I believe in Oregon . The guy was a walking code book. Its really nice to get an instructor who can realyy teach an otherwise dry subject.

 

[rbalex]

Re: Fuel Pump Disconnect

Let us assume the circuit is otherwise wired correctly but using a disconnect on the "hot" only.

Are you under the impression that with the disconnect open an arc can be created between the grounded conductor and ground?

 

[don_resqcapt19]

Re: Fuel Pump Disconnect

rbalex,

Are you under the impression that with the disconnect open an arc can be created between the grounded conductor and ground?

It's not an impression, it's a fact if the circuit does not originate at the service disconnect or source of a SDS.
Don

 

[rbalex]

Re: Fuel Pump Disconnect

Ah, I see your point. If the circuit originated in a sub-panel rather than the service or SDS the grounded conductor between the sub-panel and its source could still have a voltage difference to ground created by other circuits originating in the sub-panel.

Thanks for the reminder that I need to read and understand the entire response.

 

[don_resqcapt19]

Re: Fuel Pump Disconnect

rbalex,
Yes, there will be a voltage drop on the grounded conductor feeder between the service panel and the subpanel. This voltage drop will appear between the grounded conductor and a grounded object at the dispenser.
Even if the circuit originates at the service disconnect it is still possible to produce an arc by touching the grounded conductor to a grounded object at the dispenser, but it is not as likely. This can create a parallel path for grounded conductor current in some cases.
Don