Why would Line-Earth and Line-Neutral MOVs have different ratings? - Page 1
Forum section: electronics / projects

author: eecook
author_rank: Regular Contributor
timestamp: November 06, 2017, 09:32:35 pm
content: So, I took apart a microinverter and found a typical arrangement of MOVs for surge protection (Line-Neutral, Line-Earth, Neutral-Earth).Here's the info of the MOVs:Line-NeutralPN: ERZ-E14A431Voltage: 430VPeak Current (8/20): 10kALine-Earth (and Neutral-Earth)PN: ERZ-E08A751Voltage: 750VPeak Current (8/20): 3.5kAThe only reason I can think of for having a more robust MOV between L-N I can think of is that it will be subject to more stress since only that pair of wires will experience load switching and temporary over voltages. That could be a reason to raise the current rating as it will be able to withstand more transient events, but why is the voltage rating 750V for the other two? I have no idea.I would love to hear thoughts on this...Thanks!

author: Cerebus
author_rank: Super Contributor
timestamp: November 06, 2017, 10:41:36 pm
content: One thought that comes to mind is that the designer might have use cases in mind where the Neutral<->Earth voltage can't be assumed to be zero or a few volts at most. So the potential Live<->Earth voltage could be the sum of the Live<->Neutral and Neutral<->Earth. That doesn't explain having two higher voltage MOVs, but BOM reduction might.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2017, 04:03:30 am
content: Not IEC 61000-4-5, but the one that calls it out (-6-something, depending on which market, and type of equipment, you have), allows different surge impedances for line-line and line-ground. This is justifiable as most mains systems have a low impedance from line-to-line (a transformer -- which, when struck by lightning, will shunt most of the energy across itself, leaving a residual line-line blip that is short, comparable to the 8/20us current pulse specified in -4-5). In contrast, the line-ground path is longer and higher impedance. Usually, neutral is grounded at the panel or pad (transformer), giving a higher impedance (longer and thinner wires, soil impedance) and longer pulse duration, comparable to the 1.5/50us voltage pulse.Long signal lines, that may see induced lightning, are tested similarly, where in this case the extra resistance is justifiable because of the smaller conductors. Other standards may also be applied (telecom, etc.), where a longer pulse may be used (10/1000us, say). (IEC 60950-1 defines these as TNV type circuits.)Speaking of 60950 -- a single MOV to GND may not be acceptable (which is always the case under 60950, IIRC -- but again, different standards for different markets and products), because of ground leakage current caused by breakdown of the component (MOVs are progressively damaged by surges). For this standard, you use an MOV in series with a GDT. The GDT is normally off, but becomes a very low voltage drop when a high voltage is applied. (You can't use the GDT alone, because mains voltage may continue conduction until the GDT explodes.)Two protective devices in series results in a higher peak impulse voltage, but that's acceptable because mains circuits are tested to 1.5kV or more, across the isolation barrier (common mode, line-ground).So, what exact standard they're meeting with the OP's reported configuration -- I don't know, but the above shows some rationale for choosing higher voltage parts for the common mode.Tim

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 07, 2017, 04:40:09 am
content: The vast majority of surge protectors I have opened up use MOVs hot to neutral as well as hot/neutral to ground. In fact, I have only opened up one that used a GDT.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2017, 06:28:29 pm
content: They also use thermal fuses to disconnect leaky (hot) MOVs, helping to preventing fire as well.You should see GDTs in medical surge protectors and some professional data equipment, as that's the primary application of 60950-1.Tim

author: floobydust
author_rank: Super Contributor
timestamp: November 07, 2017, 06:57:29 pm
content: In North America, electrical code requires neutral and earth-ground be bonded together at the service entrance (breaker panel).So you wouldn't see huge differences (transient voltages) between them within a building. I think mostly L-N transients are the problem, not N-EGND.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 07, 2017, 08:02:47 pm
content: Indeed, the average should be very small.Don't leave out the possibility for N-GND faults (it can happen), or poor soil impedance, or sheer distance. 100m across a shop is a big antenna for induced surge from nearby lightning. Local GND is often used (e.g., outlets bolted into steel framing), which will have a different potential (whether through induced EMF in the structure, or in the ground due to its impedance) than the wired GND return path, which will also be a poor ground at these time distances and scales (10s microseconds).On the upside, the obligatory use of cable and conduit (to code) means a minimum of induction between those lines (small loop area). So that, except for faults to environment ground (where a GFCI/RCD acts, and where the loop is large), the current is always carried and returned within the cable/conduit, and except when measured with respect to environment ground (where the loop is large), the induced potential is fairly small.Tim