GMI spec - UL 2231-1 ?

[Nick Sayer]

Damn UL and their hidden standards.

Does anyone know what the minimum trip level is for ground impedance monitoring of an EVSE?

I’m fixin’ to put a potentiometer on the ground for mine and see what the resistance is that causes a grounding fault to be registered. It’d be super nice to know what UL requires for comparison.

[EV@TucsonEV]

My understanding is that since they are used in a garage, it is 20ma.

Rush Dougherty
Tucson, AZ 85719

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[Nick Sayer]

No, that's the GFCI trip current. I'm talking about the GMI - the maximum ground impedance.

[Nick Sayer]

Well, I finally sat down and performed the actual experiment.

At L1, it takes an impedance of around 120k to set off the GMI system of OpenEVSE II.

I can't imagine that's sufficiently sensitive to pass UL 2231-2. A damp human would offer far less resistance than that.

Chris, have you attempted this test with the MID-400 based system?

I'm worried that UL wants to see some absurdly low impedance, like 1k max. I had a commenter on my blog contact me with doubts that such an impedance could be detected using our methods. The impedance levels we're attempting to test are well under 1% of the series resistance already present.

> On Jan 13, 2015, at 6:49 PM, Nick Sayer <nsa...@kfu.com> wrote:
>
> No, that's the GFCI trip current. I'm talking about the GMI - the maximum ground impedance.
>

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[Jean-Michel Olivier]

In France, the maximum ground impedance for home setup is 100 Ω (Standard NF C 15-100).

The Renault Zoé checks that ground impedance is less than 150 Ω.

[Nick Sayer]

Yeah, that makes sense. I've got no idea how I'm going to make an opto-isolated test that sensitive. 

Sent from my iPhone

On Jan 14, 2015, at 11:35 AM, Jean-Michel Olivier <jeanmiche...@gmail.com> wrote:

In France, the maximum ground impedance for home setup is 100 Ω (Standard NF C 15-100).

The Renault Zoé checks that ground impedance is less than 150 Ω.

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[ILYA Shu.]

Hi Nick, 

I have exact same problem now... Did you find an answer for your question? (what ground impedance is not sufficient according to UL?)

Thanks in advance,

Ilya

[Nick Sayer]

I still haven’t gotten a chance to try my latest design. That design uses an isolated 5v supply to power an active circuit that tries to accurately measure a small leakage current and send a binary answer out through the opto-isolator.

Sent from my iPhone

[ILYA Shu.]

Thank you for quick reply.

I have also designed several circuits that could work (10kohm impedance detection is much more straightforward and easy implementation) . But I was unable to get an answer from ul2231 standard, as well from certification lab regarding allowable ground impedance value in ohms...

The other issue is HiPot testing with such a circuit, but this is a bit different story. 

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[Nick Sayer]

The maximum allowable impedance is 100 Ω per volt, so 12k for L1 and 20-24k (depending on supply) for L2. 

Sent from my iPhone

[ILYA Shu.]

Nick, 

As far as I know this rule is related to Isolation monitor and not ground monitor. Can you please tell where this 100 ohms per volt limitation comes from? (which standard etc...)

Thanks, 

Ilya

[Nick Sayer]

I’m flying to Vegas for DEFCON this week, but I am pretty sure this was part of the UL EVSE spec.  In addition, I’ve heard independent confirmation from Chris that that’s what he’s heard from UL people. Maybe he can chime in on this. 

Sent from my iPhone

[chris1howell]

I have not read anything about a specific trip point for GMI.... A customer went through GMI testing about 6 months ago and noted the test was performed by adding a 1M Ohm resistor to the input. that when switched in would trigger the GMI Fault. 

[Nick Sayer]

The 100Ω per volt was such a specific value. I remember it from *somewhere*. What else could it be?

Now it’s going to bug me.

[ILYA Shu.]

Hi Chris, 

Sounds interesting! Do you know where the test was conducted? (what certification lab) 

Nick, 

Wow, DEFCON should be lots of fun! 

Thank you, 

Ilya

[Sam C. Lin]

Nick,

You got the 100 ohms/volt from UL2231-2:

 

http://www.intertek.com/standards-updates/ul-2231-2-personnel-protection-systems-supply-circuits/

 

The Manufacturer’s Proprietary Test Program for IM/I shall include means to verify correct operation of the IM/I functions of 100 percent of production of systems incorporating an IM/I. The testing shall verify that the IM/I inhibits the application of power to the load when the resistance from any ungrounded conductor to ground is less than 100 ohms/volt based on the nominal system voltage. The testing shall also verify that the IM/I will interrupt the power to the load when the resistance from any ungrounded conductor to ground becomes less than 100 ohms/volt based on the nominal system voltage.

 

-Sam

[Nick Sayer]

Huh. 

Maybe all of my fussing about it is a waste of time. Certainly you’d expect the normal impedance from a conductor to ground to be far higher than that.

So what would be a reasonable maximum allowable (GCM trip point) impedance to ground?

If I’m reading properly, they’re testing with a megaohm (according to Chris earlier in this thread), but that seems way too high to be actually allowed. 

Sent from my iPhone

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[ILYA Shu.]

Hi Nick,

Yeah, 100Ohms/volt are coming from IM (Insulation monitor) like Sam mentioned... 

Since there is no place in standard where they actually mention "acceptable level" of ground impedance. The only "measure" of impedance is that it's not allowable to be more than 150Vrms to PE in the location of "fault" (From standard UL2231-1). If you define your product to be split-phase 120Vrms there is no way it will be more that 150Vrms in the fault point. So, seems like even 1MOhm is sufficient according to standard. 

Best regards,

Ilya

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