gfi's

[hockeyoligist2]

I think I may have got some education yesterday. Per ( http://www.mikeholt.com/code_forum/showthread.php?t=78681 ) I'm not real familiar with gfi"s since i'm industrial and we don't use them a lot. but when I googled it i found that a gfi will not normally trip with a short between the hot and the grounded conductor......... OK,.... I don't understand this......... I know it is to protect someone from shock...... but it will not protect from fire? Am I reading this wrong or are they not going to trip in that situation? Someone explain please? I'm more worried about fire than shock. I take care when I'm running power tools etc. but a fire could come in the middle of the night and if I've had a few ML"s then I would die?

 

[iwire]

A GFCI will not sense a hot to neutral (actually any line to line fault) conductor fault, it 'sees' that as a normal load. The breaker protecting the circuit has to handle.

If you grab a hot in one hand a neutral in the other hand of a GFCI protected circuit you will be in tough shape.

 

[augie47]

gfi's

In the field I have seen them trip on a short condition, however, you need to remember what they are designed for: a "fault to ground" or an unbalanced condition. A line to neutral short is not an imbalance, just a BIGGG load.
Overload devices protect from overload, short circuit devices from shorts and gfis from ground fault.

 

[charlie b]

. . . A line to neutral short is not an imbalance, just a BIGGG load.

Well put, Augie. I've never seen it stated so simply and eloquently.

 

[LarryFine]

but when I googled it i found that a gfi will not normally trip with a short between the hot and the grounded conductor......... OK,.... I don't understand this......... I know it is to protect someone from shock...... but it will not protect from fire? Am I reading this wrong or are they not going to trip in that situation?

A stand-alone GFCI device, such as a receptacle, a faceless unit, or a plug-in device, will only trip under the imbalanced conditions they're meant for, as when the current in the hot and the neutral are different by 5ma or more.

However, a GFCI breaker incorporates both a GFCI circuit and a standard over-current/short-circuit protective device, and will trip under any of the conditions they're designed for.

Any properly-wired circuit will be protected against all of the conditions necessary, whether by two separate devices or a single device.

 

[hockeyoligist2]

Ok, so that means that a direct short between the line and grounded conducter will not cause the breaker to trip? So it protects someone in the bathtub with a hair dryer. But not someone asleep in the bed thats not doing something dumb? Unless I'm misunderstanding that seems real dumb. I think more people die from electrical fires than from hairdryers in the tub? I'm showing my stupidity here............ since I don't know a lot about them. But, it seems to me they should trip on over currant as well?

I'm just here to learn, so please excuse me!!!!!!!!!!

 

[roger]

Ok, so that means that a direct short between the line and grounded conducter will not cause the breaker to trip?

Yes the breaker will trip, but it will be due to overcurrent, not an imbalance of current that will trip a GFCI.

Roger

 

[iwire]

Ok, so that means that a direct short between the line and grounded conductor will not cause the breaker to trip?

Your still missing it.

Lets make sure we are talking about the same thing.

Lets ignore GFCI receptacles and just talk about GFCI breakers.

GFCI breaker has a section to handle line to line overloads just like any breaker.

That is the part that protects against fires caused by overheated conductors and equipment.

Additionally a GFCI breaker has ground fault sensing.

Here is a low tech description of how it works.

This ground fault sensor basically works by comparing the current leaving the breaker with the current returning to the breaker.

If the outgoing current does not match the incoming current it is said to be 'imbalanced' the GFCI sees this imbalance and assumes the imbalance is caused by a ground fault.

Once the imbalance is greater than about 5 ma the GFCI will open the circuit.

 

[hockeyoligist2]

OK, then a sump pump that is stopped up on the intake side as in http://www.mikeholt.com/code_forum/showthread.php?t=78681 that may be drawing too much current wouldn't trip? The ones that I have worked with are on normal breakers, they get clogged up and trip from over currant. Clean them and they are fine. Acording to the pump schools, (gorman rupp) that I have been to you can restrict the outgoing and the amps will drop, restrict the intake and the amps go up and they also overheat from cavitation. If a GFI does protect for over currant then it should trip also?

"I'm learning" Slooooooooooooowly

And thanks for all the help!

 

[charlie b]

If a GFI does protect for over current then it should trip also?

Saying "GFI" is not enough. We need to know if you are talking about a GFI receptacle served by a normal (i.e., non-GFI breaker), or if you are talking about a normal receptacle served by a GFI breaker.

But in either case,

(1) If you have overcurrent, the breaker will trip on overcurrent. A GFI device will not be involved in of the trip.

(2) If you have a situation in which the GFI is tripping, then this is not an overcurrent situation. You have a failed "somethingorother." It might be the pump. It might be the permanent wiring. It might be the portable cord. It might be the receptacle. It might be the breaker. I don't know. All I know is that for a GFI to trip, there must be some current going back to the source by some path other than the grounded (i.e., neutral) wire. That's bad; that must be fixed.

 

[LarryFine]

Hockey, you're still still not getting it. You're confusing the two different types of protection:

1. Overcurrent and short-circuit protection; standard breakers and fuses protect electrical systems, wiring, and sometimes electrical equipment. This is what the stopped-up sump pump will trip.

2. Ground-fault protection for personnel; part of a breaker or a receptacle. GFCI protection is provided solely to protect people from electric shock, and practically zero protection for equipment and wiring systems.



GFCI devices can be either built into a breaker, which provides both types of protection, or they can be separate devices, like GFCI receptacles. The latter circuit's standard circuit breaker will provide the protection you're asking about, regardless of GFCI protection.

 

[haskindm]

Hockey,
I believe you are also confused about what you learned from the pump manufaturer. If you restrict the outgoing, the amps should go up as the pump will be working harder. If you restrict the intake the amperage should be less as there is nothing for the pump to "pump". Try it at home with your vacuum cleaner. Block the intake and you will hear the motor speed up. An amp clamp would show a reduced amperage. There is no air for the vacuum to pump, so it is doing less work.
In your original post you mention GFCI as protecting against fire. GFCI protection for personnel is intended to protect people from electrocution. It will not prevent shock, but the shock should not be fatal (unless it causes you to fall off a tall scaffold or ladder). GFI protection for equipment may be some protection against fire. The relatively new AFCI devices are designed to prevent fires. There is some controversy over their effectiveness, but that is the purpose.
Keep asking questions and learning.

 

[hockeyoligist2]

Hockey,
I believe you are also confused about what you learned from the pump manufaturer. If you restrict the outgoing, the amps should go up as the pump will be working harder. If you restrict the intake the amperage should be less as there is nothing for the pump to "pump". Try it at home with your vacuum cleaner. Block the intake and you will hear the motor speed up. An amp clamp would show a reduced amperage. There is no air for the vacuum to pump, so it is doing less work.
In your original post you mention GFCI as protecting against fire. GFCI protection for personnel is intended to protect people from electrocution. It will not prevent shock, but the shock should not be fatal (unless it causes you to fall off a tall scaffold or ladder). GFI protection for equipment may be some protection against fire. The relatively new AFCI devices are designed to prevent fires. There is some controversy over their effectiveness, but that is the purpose.
Keep asking questions and learning.

Thanks for your response, I think I misread the post that was on the gfi tripping, and I have learned a lot about "Overall" ground fault protection. I know that a receptacle that's is ground fault protected will trip on a ground fault and I know that if it is on a regular circuit breaker the circuit breaker will trip on an overload situation. I just wasn't sure if a ground fault breaker would trip due to over load from what I was reading. Thanks to everyone that responded and cleared that up!

OK, now for the pump.........When dealing with Variable Torque loads things are not always as they would seem. The amperage will go up if you restrict the intake, as a test I've closed valves on both the intake and the exhaust on pumps with an amprobe connected and the amps stay the same (or drop) on the exhaust. But they go up when the intake is restricted. Yes the motor will speed up on the vacuum cleaner,(or a pump) but it is working harder. with centrifugal water pumps it is more obvious because of the noise if you close off the outgoing it is happy close or restrict the incoming it gets real noisy! maybe this site will explain? http://www.motorsanddrives.com/cowern/motorterms8.html

No offense meant but since I have to deal with pumps a lot, I know at least enough to get me in trouble I just don't know a lot about the non-industrial side

52 and still learning!

 

[don_resqcapt19]

hockeyoligist2,

The amperage will go up if you restrict the intake, ...

Your cited source does not support this statement.

The second puzzling thing that occurs with variable torque loads is that the motor load actually decreases as the output or input to the blower or pump is blocked off or restricted.

Don

 

[hockeyoligist2]

Hmm. I didn't read it all had a call when I posted. I'll dig out my Gorman Rupp books and scan them.

 

[LarryFine]

The average fan or impeller will exhibit a reduced load when the intake, or sometimes even the outlet, is blocked, because the medium being moved will just spin around with the blade or impeller, which is easier to do than deing pumped down the line.

 

[hardworkingstiff]

What kind of pumps are we talking about? Positive displacement, centrifugal, or ????

 

[LarryFine]

(Obviously, my blanket statement would not apply to positive-displacement equipment.)

 

[hillbilly]

Restricting the intake is not a good way to regulate the flow of any pump, be it centrifugal or positive displacement. A centrifugal pump uses centrifugal force to move liquid and is sized (selected) to pump a certain amount of liquid at a certain volume and against a certain back pressure. The selection process includes fluid viscosity, desired flow rate, and calculated back (head) presure that will oppose the flow. At maximum flow, the pump will require maximum horsepower. If the flow is restricted (closing valve, increased head pressure, increased piping loss such as elbows and valves) the flow and the horsepower required will decrease simultaneously. If the discharge valve is closed completely, the horsepower (electrical current if a electric pump) will decrease dramatically although the pump pressure will climb to it's maximum amount (cut-off pressure). As the valve is slowly opened, the flow rate and required power will increase simultaneously until it reaches it's (most efficient) maximum flow. Any further decrease in back pressure will cause greater flow at decreased efficiency. Restricting the intake will cause reduced flow but also cause cavitation (starved suction) in the impeller and result in noise and damage to the impeller and housing.
A positive displacement pump is just what it says. Flow rate is dependant upon pump displacement, fluid viscosity, and power input. Any incease in pump speed will result in increased pressure, fluid flow and required input power. The power increase is dependant on the fluid weight of the increased flow and the resulting resistance to flow caused by pump and piping loss (increased resistance to flow). If you close the discharge valve on a positive displacement pump, the flow will decrease and the discharge pressure and required power will increase until the power source (motor) is overloaded or something breaks. To prevent this, most positive displacements pumps have a relief valve to allow excess fluid (pressure) to be diveted back to the intake (re-circulated). Restricting the intake on a positive displacement pump can also cause pump damage.
I learned this by experience, so some of it may be questioned by any experts out there. I'm open for discussion and willing to learn (or teach).
steve