Why the Limit Here?

[hillbilly1]

What values did you get? What was the lowest?

636 and 650 amps. Within 10’ of the panel, panel about 20’ from the meter, about 100’ run to a pole mount transformer.

 

[synchro]

Can't the results of a voltage drop tester such as the Ideal SureTest be worked mathematically to give an available fault current at that point on the circuit? That device provides the following voltage drop testing.
% Voltage drop under load (12A, 15A, 20A load tests): 0.1% - 50.0%

You could get the L-N fault current from that test. I guess you could also measure the L-G voltage drop after making a bootleg ground connection to the tester's neutral, and then calculate the L-G fault current (if you are adventurous enough to do that). Of course if there's a GFCI breaker on the circuit that would pose a problem unless you replaced it with a regular breaker at least temporarily.

It may be something that could be done if a problem is suspected, but too much messing around to do for every branch circuit I think.

 

[mbrooke]

636 and 650 amps. Within 10’ of the panel, panel about 20’ from the meter, about 100’ run to a pole mount transformer.

Interesting, thanks! That gives you a Ze of about 0.185 ohms. Well within limits.

 

[mikeames]

Not a job for a megger, which measures high resistance values at high voltages. Instead you need a device to measure low resistances, often a four terminal measurement.

I understand but some of the meggers have this test option built in from what I have been told.

 

[mikeames]

I understand but some of the meggers have this test option built in from what I have been told.

Or perhaps it was in reference to the megger brand

 

[mbrooke]

Or perhaps it was in reference to the megger brand

This is how you do it:

 

[mbrooke]

Better vid, but its not US:

 

[paulengr]

I like the idea but it’s too late. It would be good at the panel for AIC. But on branch circuits the big deal is grounding issues but not sure how to test with the latest Code.

In residential it will trip the AFCIs and GFCIs every time. Most smaller molded case breakers under 50 A or so have instantaneous set to trip in 1 cycle.

Only practical way would he say to use say 5 x breaker size. So to avoid a 15 A breaker use 5x15 = 75 A so around a 120/75 = 1.6 ohm resistor. The neutral test will be fine but the ground test will trip a GFCI.

On the ground side can’t we use the same method as a clamp on ground tester? So inject say 20 KHz and measure the impedance from L to G.

As to length issues look at the free “GEMI” software and documentation. Line length is a big deal that is largely ignored. The steel tubing institute is the big promoter because it inherently promotes using conduit but never got traction.

 

[mbrooke]

I like the idea but it’s too late. It would be good at the panel for AIC. But on branch circuits the big deal is grounding issues but not sure how to test with the latest Code.

In residential it will trip the AFCIs and GFCIs every time. Most smaller molded case breakers under 50 A or so have instantaneous set to trip in 1 cycle.

Only practical way would he say to use say 5 x breaker size. So to avoid a 15 A breaker use 5x15 = 75 A so around a 120/75 = 1.6 ohm resistor. The neutral test will be fine but the ground test will trip a GFCI.

On the ground side can’t we use the same method as a clamp on ground tester? So inject say 20 KHz and measure the impedance from L to G.

As to length issues look at the free “GEMI” software and documentation. Line length is a big deal that is largely ignored. The steel tubing institute is the big promoter because it inherently promotes using conduit but never got traction.

My understanding is that this device has a current limiting resistor and takes current in "pulses" so it doesn't trip a standard breaker or an AFCI without GFP. So it should will in that regard.

The issue needs to be addressed, because the CMPs is using this as a means to justify not only AFCIs but also GFCIs and GFPs.

 

[don_resqcapt19]

My understanding is that this device has a current limiting resistor and takes current in "pulses" so it doesn't trip a standard breaker or an AFCI without GFP. So it should will in that regard.

The issue needs to be addressed, because the CMPs is using this as a means to justify not only AFCIs but also GFCIs and GFPs.

This issue has ZERO to do with GFCI requirements.

 

[mbrooke]

This issue has ZERO to do with GFCI requirements.

In the past, today in play a major part in the expansion of GFCIs/GFP.

Consulting - Specifying Engineer | UL’s new GFCI classes

Consulting - Specifying Engineer - Learning objectives Understand UL’s new GFCI classes. Understand how GFCIs for 240 to 600 V applications differ from the familiar Class A GFCIs. Know

www.csemag.com

 

[mbrooke]

This issue has ZERO to do with GFCI requirements.

From my link above:

Class C, D, and E GFCIs trip at 20 mA rather than the 6 mA trip current mandated for Class A GFCIs. This increase in GFCI trip level is allowed by UL assuming the availability of a reliable ground in parallel with the body. During a fault, the grounding conductor will shunt the fault current around the body and cause the device to trip. This provides the let-go protection, while the 20 mA threshold provides protection against fibrillation. (If there is no grounding conductor, such as in two-wire household products, then the GFCI must provide both let-go and fibrillation protection, and a Class A device is required.) See Figure 1.

If the assumption is that an effective ground fault current path will be available, then why have a 20 milli amp differential in the first place? According the UL's body graph (which is a direct copy of IEC TS 60479-1, which table 41.1 is based off of), a 0.4 second disconnection time will prevent fibrillation.

Which takes us to the NEC. Where in the NEC is a circuit over 150 volts to ground mandated to disconnect within 0.4 seconds during a ground fault? Where in the NEC are branch circuit limits imposed to assure a 0.4 second disconnection time?

Nowhere. In fact a 90 second ground fault trip time will satisfy the NEC. At 10 seconds in you're talking electrical burns.

It would be nice if the CMPs could swallow their pride for a moment.

 

[mbrooke]

Table 41.1:



This is what UL is basing there graph off of, can you feel the de ja vu?

File:IEC TS 60479-1 electric shock graph.svg - Wikimedia Commons

commons.wikimedia.org

The CMPs know they screwed up by never defining an effective ground fault current path, with manufacturers being aware of that blunder as well. Instead of making a table or rule that would increase copper (or AL) in a small percentage of installations they are now indifferently expanding GFCI/GFP requirements to encompass all circuits.

Mandating only one solution where others would fulfill the same intended goals is in of itself implying malfeasance.

 

[don_resqcapt19]

From my link above:




If the assumption is that an effective ground fault current path will be available, then why have a 20 milli amp differential in the first place? According the UL's body graph (which is a direct copy of IEC TS 60479-1, which table 41.1 is based off of), a 0.4 second disconnection time will prevent fibrillation.

Which takes us to the NEC. Where in the NEC is a circuit over 150 volts to ground mandated to disconnect within 0.4 seconds during a ground fault? Where in the NEC are branch circuit limits imposed to assure a 0.4 second disconnection time?

Nowhere. In fact a 90 second ground fault trip time will satisfy the NEC. At 10 seconds in you're talking electrical burns.

It would be nice if the CMPs could swallow their pride for a moment.

There are plenty of opportunities for shock that do not involve an effective ground fault current path.

 

[mbrooke]

There are plenty of opportunities for shock that do not involve an effective ground fault current path.

How do you define an effective ground fault current path though? If you mean direct contact, sure. Yet thats not ULs or the CMP's reasoning behind GFP and GFCIs in many locations.

 

[mikeames]

From my link above:

Nowhere. In fact a 90 second ground fault trip time will satisfy the NEC. At 10 seconds in you're talking electrical burns.

It would be nice if the CMPs could swallow their pride for a moment.

I agree with Don above. You seem to be swaying in the direction of protecting people. Although there are articles and devices designed to protect people, much of the NEC is to prevent equipment damage and fire. So in my opinion a 90 second fault is fine if it wont cause fire and destruction. If your concern is human safety then perhaps there's a better alternative then fault impedance.

 

[mbrooke]

I agree with Don above. You seem to be swaying in the direction of protecting people. Although there are articles and devices designed to protect people, much of the NEC is to prevent equipment damage and fire. So in my opinion a 90 second fault is fine if it wont cause fire and destruction. If your concern is human safety then perhaps there's a better alternative then fault impedance.

A 90 second fault can certainly cause a fire considering the EGC is smaller in many scenarios.


GFCI/GFP is not a better alternative, far from it.

Also I disagree- the NEC is not in place to protect equipment minus a select few scenarios which are linked to life safety ie MOVs in emergency panel boards.

 

[don_resqcapt19]

....
Also I disagree- the NEC is not in place to protect equipment minus a select few scenarios which are linked to life safety ie MOVs in emergency panel boards.

Equipment falls into the broad definition of property and the purposes of the code is "protect persons and property" from the hazards of electricity.

 

[don_resqcapt19]

How do you define an effective ground fault current path though? If you mean direct contact, sure. Yet thats not ULs or the CMP's reasoning behind GFP and GFCIs in many locations.

If you go back in time, the GFCI protection was all for cord and plug connected equipment. One of the driving factors was the fact that it is much more common for the EGC to fail on cord and plug connected equipment than on hard wired equipment. That is still the case. GFCI provides people protection for cord and plug connected equipment with a ground fault and a failed EGC. No testing of the fixed wiring will prevent damage to the EGC in the cord.

 

[paulengr]

How do you define an effective ground fault current path though? If you mean direct contact, sure. Yet thats not ULs or the CMP's reasoning behind GFP and GFCIs in many locations.

Not only that but even in a situation where it’s all an effective path...e.g. a totally nude body on a metal deck touching a grounded metal surface, it takes SIGNIFICANT current considering that one very common mistake with the IEC data is that human resistance drops to 600 ohms minimum depending on applied voltage (its nonlinear) where at 150 V it’s around 1200 ohms or more. As compared to the more conservative constant value of 1,000 ohms used in IEEE standard 80 for substations or in MSHA mining standards. After 5 seconds if fibrillation has not occurred it won’t. A more realistic maximum is 100 mA if you plug in the numbers at even a couple seconds. The low 20 mA numbers are at 5 seconds. IEC screwed this one up royally. They mixed things together that don’t go together. Under 0.1 seconds it can’t cause fibrillation because the pulse is to short to interrupt the heart rhythm so at that point the limit is on organ damage (1 A+, based on ohmic heating) NOT fibrillation.

As an unfortunate example we used 25 A resistors on a 7200 V medium voltage system so the resistor is 166 ohms and the line to ground voltage is 4160. Tripping was set to 1-2 seconds. Mining application. With the resistor in series it will not cause fibrillation. Due to confusion over which system he was on and some serious LOTO issues a neighbor took a hand to knee hit on this system long to ground and lived to tell about it.