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Author Topic: GFI questions  (Read 8590 times)

TJ (Tom) Cornish

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Re: GFI questions
« Reply #10 on: November 06, 2013, 08:22:02 am »

Tom,

The 2008 NEC started requiring CAFCI (Combination AFCI) kind of off topic to explain that-but depending on a lot of variables you could certainly have an older design-I have even heard of non-combination winding up on big box store shelves in the last 2-3 years even though they no longer meet code.  I think 2005 was the first code cycle to require AFCI-so 2007 would be the early days.

As for doing strange things-keep in mind that AFCI's will trip on a ground to neutral short as well as a ground to hot or hot to neutral-so if you have any home brew electronics that have neutral the to the chassis as was often done, and if a 3 wire power cord has been added the AFCI will be unhappy.

I don't always agree with the code requirements-just have to understand and live with them.  I will say though, that as much as I hate to admit it, more often than not I eventually learn that someone was smarter than me when they made the rule!

Steve Swaffer
Mine are combination type, Eaton/Cutler Hammer.  One was defective out of the box - wouldn't trip when test button was pressed. 

I have the following issues:
- running brushed motors on AFCI protected circuits doesn't work, though strangely my vacuum does. 
- With a load on one AFCI circuit in the room above my workshop, the braking action of my VFD-driven milling machine causes that breaker to trip (milling machine is on a different circuit, even a different panel than the AFCI in question) - this is a noise immunity issue that I can probably improve by installing a line filter on the VFD input wiring, but there's no arcing anywhere near the AFCI as the VFD uses IGBT switches powering a 3-phase induction motor.
- Once in a blue moon, if a kid plays with a light switch (which is brand new - not a failing switch), the AFCI detects normal small arcing inside the switch as a fault.

Perhaps they have improved since mine were made and/or maybe Eaton makes lousy AFCIs, but my experience hasn't been great.  The desired behavior of arc-fault protection does seem to be a challenge - how do you distinguish between a normal arc and a dangerous arc?  Is the future solid-state switches and brushless motors?
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Mike Sokol

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Re: GFI questions
« Reply #11 on: November 08, 2013, 02:30:57 am »

I just found this info about industrial GFCIs in Mike Holt's newsletter.

Mike Sokol

------------------------------------------------

Industrial GFCIs Are Finally Here

Contributed Article
Special to Mike Holt Enterprises Electrical News Source
October, 2013

Everyone is familiar with ground fault circuit interrupters (GFCIs). We see them in our bathrooms and kitchens at home, where they protect us from electric shock by quickly interrupting the circuit as soon as a ground fault is detected.  Simple and reliable, this technology is credited with cutting residential electrocutions in half since it was mandated by the NEC in the 70s.

A different story can be told about the workplace. From 1992-2002, there were 3,348 electrocutions on the job. Despite higher voltages in the workplace that increase the danger of shock, only recently has GFCI technology become available for industrial and commercial settings. The first UL Listed GFCI for applications up to 600 V became available last year.

What is an Industrial GFCI?

An industrial GFCI (UL calls it a “special purpose” GFCI) operates like any other, except for some important differences. First, the trip threshold is set at 20 mA, higher than its residential cousin. Second, the GFCI device must monitor the ground wire for open connections, so that if the connection to ground is lost, then the device will open the circuit. Third, an industrial GFCI interrupts higher power, so it must be much larger. It mounts like an electrical panel or inside an existing electrical cabinet.

According to the UL 943 standard, Class A GFCIs (for use on circuits up to 240 V) must trip at 6 mA. Unfortunately these devices cannot be used in industrial facilities, which typically have significant ground leakage currents that would cause nuisance tripping.

In 2000, UL addressed GFCIs for higher voltage applications with a draft standard “Outline of Investigation” called 943 C and added to it in 2009, but no commercially available product could pass its test requirements until recently. UL 943C Class C, Class D, and Class E define the characteristics expected of a GFCI operating up to 600 V.

Class C – For circuits with no conductor over 300 VAC to ground where reliable equipment grounding or double insulation is provided.

Class D – For in circuits with one or more conductors over 300 volts to ground, and with specially sized, reliable grounding, to provide a low impedance path so that the voltage across the body during a fault does not exceed 150 volts.

Class E – For circuits with one or more conductors over 300 volts to ground but with conventional equipment grounding provided for the protected equipment in the system or double insulation. These devices respond rapidly to open the circuit.

Why the 20mA trip threshold?

UL 943C sets the trip threshold of industrial GFCIs at 20 mA — low enough to provide worker shock protection but high enough in most applications to avoid nuisance tripping. Like household GFCIs, their response to ground current follows an inverse-time curve; a current of 20 mA will cause a trip in about 1 s (quickly enough to prevent injury at this current level), and higher currents will cause a trip within 20 ms.

In some cases, the 20 mA trip threshold may still cause nuisance tripping. One option is an equipment ground-fault protection device (EGFPD), which is like a GFCI except it has adjustable trip settings from 6 to 50 mA.  Although EGFPDs protect workers against shock, UL lists them in a category for equipment protection rather than for people protection.

Where are industrial GFCIs used?

Wet applications deserve scrutiny, because water can increase the risk of shock by bringing workers into contact with a ground fault. Industrial GFCIs can be applied to submersible pumps, wet saws, and process equipment such as augers and mixers that handle wet material. Review damp or wet work areas, such as those found in food processing facilities, and areas where equipment is subjected to washdown cleaning.

Industrial GFCIs can be mounted on a wheeled cart and used to protect temporary applications, especially outdoors where electrical equipment is exposed to moisture.

Also consider any application wet or dry where workers come into contact with equipment operating at high voltage, such as welding receptacles.

Long overdue

This is a safety technology that has been long overdue. Now that UL Listed industrial GFCIs are available, the adoption of such protection will grow. Industrial GFCIs are not required by the electrical code, but if the history of residential GFCIs are a model, then the code may be updated to require GFCIs in more industrial applications.

More information is available in a white paper from Littelfuse: http://www.littelfuse.com/~/media/Files/Littelfuse/Technical%20Resources/Documents/White%20Papers/Littelfuse_Industrial_Shockblock_GFCI_WhitePaper.pdf

Media Contact: Mark Johnson
Goldstein Group Communications
440-914-4700
mjohnson@ggcomm.com

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Lyle Williams

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Re: GFI questions
« Reply #12 on: November 22, 2013, 05:28:57 am »

In Australia with a 230VAC 50Hz TNCS system regular RCDs are 30mA.  Better RCDs are dual pole 30mA.  Hospital RCDs are dual pole 10mA.

Nuisance trips can still happen at 30mA, but generally only where wiring is poor, wet, or where one RCD covers a whole house.

We have a standard AS/NZS3760 that calls for periodic testing of portable RCDs.  Trip times at rated current are tested, but haven't seen one fail.  The shortest trip times I see are about 8ms, and the longest about 31ms.

While it is hard to attribute outcomes to particular initiatives, the widespread use of 30mA RCDs plus regular inspection/tagging of workplace electrical equipment plus education campaigns about overhead wire risks has halved the number of electrocutions in Australia.

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Lyle Williams

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Re: GFI questions
« Reply #13 on: November 30, 2013, 01:59:50 am »

In Australia with a 230VAC 50Hz TNCS system regular RCDs are 30mA.  Better RCDs are dual pole 30mA.  Hospital RCDs are dual pole 10mA.

Nuisance trips can still happen at 30mA, but generally only where wiring is poor, wet, or where one RCD covers a whole house.

We have a standard AS/NZS3760 that calls for periodic testing of portable RCDs.  Trip times at rated current are tested, but haven't seen one fail.  The shortest trip times I see are about 8ms, and the longest about 31ms.

While it is hard to attribute outcomes to particular initiatives, the widespread use of 30mA RCDs plus regular inspection/tagging of workplace electrical equipment plus education campaigns about overhead wire risks has halved the number of electrocutions in Australia.

Well, it looks like I cursed myself by saying no failures.  Some loaned gear came back in.  The RCD in a network controlled power board is dead.  The gear had no problems in operation, but the safety switch no longer offers any protection.
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Rob Spence

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Re: GFI questions
« Reply #14 on: November 30, 2013, 11:25:39 am »

Mine are combination type, Eaton/Cutler Hammer.  One was defective out of the box - wouldn't trip when test button was pressed. 

I have the following issues:
- running brushed motors on AFCI protected circuits doesn't work, though strangely my vacuum does. 
- With a load on one AFCI circuit in the room above my workshop, the braking action of my VFD-driven milling machine causes that breaker to trip (milling machine is on a different circuit, even a different panel than the AFCI in question) - this is a noise immunity issue that I can probably improve by installing a line filter on the VFD input wiring, but there's no arcing anywhere near the AFCI as the VFD uses IGBT switches powering a 3-phase induction motor.
- Once in a blue moon, if a kid plays with a light switch (which is brand new - not a failing switch), the AFCI detects normal small arcing inside the switch as a fault.

Perhaps they have improved since mine were made and/or maybe Eaton makes lousy AFCIs, but my experience hasn't been great.  The desired behavior of arc-fault protection does seem to be a challenge - how do you distinguish between a normal arc and a dangerous arc?  Is the future solid-state switches and brushless motors?

I have about 40 Siemans combination units in my house renovation from 2008. All were purchased by my electrician at electric supply houses in late 2008. No fails and no false trips in over 4 years.



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Lyle Williams

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Re: GFI questions
« Reply #15 on: November 30, 2013, 04:29:23 pm »

Just some curiosities I have,
1)  What part in a GFI is the part that wears-out and requires the "test" function that they all have?


There is some information on failure causes towards the end of this document: http://www.mikeholt.com/documents/freestuff/NEMA-GFCI-Field-Test-Survey-Report-January-2001.pdf
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Lyle Williams

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Re: GFI questions
« Reply #16 on: November 30, 2013, 05:24:36 pm »

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Re: GFI questions
« Reply #16 on: November 30, 2013, 05:24:36 pm »


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