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PV Panels Creating A Current On My Roof


jasonvanwyk

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Hi All

The other day I put a ladder up onto my roof to climb up and wash my PV panels with a hose pipe. I have a zinc corrugated roof. As I touched the ladder, I got an electrical shock. The shock was quite substantial and was not my imagination (I touched the ladder a few times to test this). Is this normal? This doesn't seem right to me. I'm sure the panels are sealed at manufacturing, so surely the panels can't be "leaking" current? My only other thought is that one of the strings must be earthing to the roof?

Any insight or comment on this would be appreciated. 

Many thanks in advance.

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12 hours ago, jasonvanwyk said:

The other day I put a ladder up onto my roof to climb up and wash my PV panels with a hose pipe. I have a zinc corrugated roof. As I touched the ladder, I got an electrical shock. The shock was quite substantial and was not my imagination (I touched the ladder a few times to test this). Is this normal? This doesn't seem right to me. I'm sure the panels are sealed at manufacturing, so surely the panels can't be "leaking" current? My only other thought is that one of the strings must be earthing to the roof?

Welcome to the experience. @plonkster  has a very nice way of explanation why this happens. Its got to do with the internal frequency of the MPPT. (Cant remember, 40KHZ or something crazy like that). 

The best solution is earthing your frames properly. Bring the Panel frames, Mounting rails Roof and ground to the same potential and that should be the end of it. 

Do a little test, connect a wire to a water steel water pipe or something that is earthed well, then touch the other end on the roof, you should see quite a nice spark. 

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12 hours ago, Jaco de Jongh said:

Welcome to the experience. @plonkster  has a very nice way of explanation why this happens. Its got to do with the internal frequency of the MPPT. (Cant remember, 40KHZ or something crazy like that). 

Physics 101. If you put an electrical conductor into an alternating magnetic field, a current is induced in that conductor. Such currents are usually referred to as "eddy currents". It's the reason why grid codes mandate that the life and neutral for all circuits must pass in the same conduit, especially when the conduit is metallic. The 50Hz alternating current in house conductors induce currents in any metal in the vicinity, but if you put both wires in the same conduit they cancel each other.

These eddy currents can of course generate a fairly unpleasant shock. It's rarely dangerous, but that is no excuse really.

Now you may wonder where the alternating field comes from given that PV modules are DC, and the answer is that it comes from the MPPT. The MPPT switches the power on and off in a square wave, usually in the 40Khz to 100kHz range, and this causes enough of a ripple to induce current in nearby conductors.

The fix is to earth the frame, and to try and keep your cables together as much as possible, ie try to avoid a situation where the positive cable runs alone along "this" side of the frame while the negative runs somewhere else.

I would expect the effect to be worse as the batteries get full. While the MPPT is tracking the maximum point there will be less of a ripple. The ripple is worse when it's limiting power.

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1 hour ago, plonkster said:

These eddy currents can of course generate a fairly unpleasant shock.

Technically its a fault voltage that we come into contact with that will drive a current through us.

Our impedance is determined by many factors such as the current path through the body, amplitude of the applied voltage, duration of the applied voltage, frequency of the applied voltage, contact surface area, pressure exerted and skin temperature.

IEC 479-1 Table 1 (for the highest risk 5 % of the population) shows that the total impedance between dry hands varies from 1750 ohms at 50 V rms to 700 ohms at 1000 V rms. The impedance of an arm is very similar to a leg. So the values mentioned also apply for the current path from one hand to one foot. The more typical situation is where the current path is from one hand to both feet and then you can use 75 % of the bove impedance value. 

So start by measuring the fault / touch voltage. 

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When I started off about a year ago i also had the roof bite me :) 

I Earthed the frames panels and roof using a copper spike of about 3m into the ground and 6mm2 earth cable. Never had a shock again.

I cant remember where i read it but someone said that i must actually drive spikes in every 5 meters and install a 16mm2 cable to them. I looked at the cost of the cables and decided i will do that when i win the lottery.

 

 

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4 hours ago, plonkster said:

and to try and keep your cables together as much as possible, ie try to avoid a situation where the positive cable runs alone along "this" side of the frame while the negative runs somewhere else.

This will also have another, related benefit. When lightning strikes nearby, a voltage is induced into the cables roughly proportional to the area between the conductors. Keeping the conductors together minimises this area, thus minimising damage from the induced voltage.

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@jasonvanwyk,  while I am fully aware of eddy currents, they don't explain your shock. They just run around in circles and make heat. Induction pots would be lethal if they were dangerous.

It is possible that it was a capacitive static charge, but that takes time to build up and you said it happened a few times in succession, so I'll rule that out too.

I assume this was a aluminium ladder with insulated feet? And a normal non-metallic hosepipe?

Your shock was because your roof is live relative to earth where you were standing. Your ladder was just the equivalent of a wire connected to your metal roof.

The DC side of solar is often unearthed, is your DC side or are your batteries earthed?

This isn't necessarily a solar issue, if your DC side isn't earthed elsewhere. You represented one earth but there has to be another earth elsewhere to make a circuit.

This is why I asked you to measure the voltage and see if its AC or DC.

We know there will be an earth on the AC.... and if I was a betting man.

See if it goes away if you trip your AC.

Edited by phil.g00
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1 hour ago, phil.g00 said:

I am fully aware of eddy currents, they don't explain your shock.

Hi Phil, this would be the first time i will disagree with you to an extend. I had the same issue on more than one installation, including my own home. I was seriously surprised when I was shocked and pulled everything apart, even replaced all my PV wires as i thought that the insulation might have broken down and allowed a leakage. My metal gutters were well earthed and if I touched the metal gutters and any of my panels I received a nice shock. Even measured  50-80 volts between earth and the frames. It was during a period of moving my panels from a ground mounted frame to my roof and they were not yet earthed. One String were operational while I was busy installing the rest, it became so unbearable that i had to solve it before i could continue.

I also suspected it was an AC leakage that might cause it, but even with the AC switched off at the Boundary box and the inverter output breaker switched off, the voltage remained. I wanted to test if it was "Ghost voltage" or real power and connected different 24 & 48 Volt coils and globes between the panel frames and earth and the moment i did that the voltage disappeared. 

In my case the eddy currents were induced in the unistrut frames. The only way I could make it stop was to break the string, Earthing everything solved it on more than one installation. 

EDIT: 6 of my panels were lying flat on wire mesh supported 2.2meters from the ground, they were part of a string of 12 panels. the other 6 were on the roof, so only a + and - wire were running from the combiner box, through the air to the panels. No earth were installed. At midday, you could not stand on the ground barefoot and touch the wire mesh at the same time. Nice serious shock. There is no other power within 10 meters from that mesh. My wife came and called me after she got shocked, i laughed at her and told her it is impossible, there was nothing that could shock her....... until I touched the mesh myself... guess who were the one laughing then? 

I thought its static and tried to discharge it by touching it first and then hold on, I could not hold on much longer than a second and had to let go. Earthed the mesh, and no more shock. 

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@Jaco de Jongh, Ah this is something different, still not eddy currents though.

Eddy currents by their very nature are current flow internal in a metal and are effectively flowing in a short circuit so there is very little voltage. 

This is induced current, but no real potential build up, think of a lump of metal as being made up of windings of a transformer but each winding being shorted out. Plenty internal current rushing about and plenty heat but no voltage build up to speak of. 

I think your dealing with the capacitive effect of an electrostatic charge which does have plenty voltage, but normally not much current.

Like the little shock you get when you touch the door handle from some carpets.

You have all the components a large plate being the panels and the roof combination, the other plate being the ground, and high frequency MPPT switching.

The higher the frequency the lower the capacitive reactance (it is inversely proportionate to frequency) so it could probably develop enough current to deliver a nice shock.

Mmm, interesting.

 

 

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1 hour ago, phil.g00 said:

Eddy currents by their very nature are current flow internal in a metal and are effectively flowing in a short circuit so there is very little voltage. 

I may have the cat by the tail here. I was taught in high school, in my electricity class (cause we had tech subjects, it was kind of a cool school, we could do that instead of biology and useless things like that), that you must put live and neutral in the same conduit otherwise the round metal pipe of the conduit acts like the secondary of a 1:1 transformer (with a really large loop). The teacher said it generates an eddy current in the pipe. I never questioned this "fact". I don't really care what you call it, I think the science is sound.

Also, heard horror stories about ham radio operators that had a neighbour that installed a new solar system... 🙂

 

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4 hours ago, plonkster said:

I may have the cat by the tail here. I was taught in high school, in my electricity class (cause we had tech subjects, it was kind of a cool school, we could do that instead of biology and useless things like that), that you must put live and neutral in the same conduit otherwise the round metal pipe of the conduit acts like the secondary of a 1:1 transformer (with a really large loop). The teacher said it generates an eddy current in the pipe. I never questioned this "fact". I don't really care what you call it, I think the science is sound.

You are correct that is exactly how eddy currents are created, by nett magnetism. It absolutely exists, no argument.

An asymmetrical AC current flow will create a nett magnetic effect, and current there and current back in close proximity  will cancel the magnetic effect, exactly what you were taught in high school.

If there is no nett magnetism and therefore no transformer effect. Your teacher was spot on, keep the ac wires together.

This is the science behind the "twisted pair".

The thing is though eddy currents although they cause losses, they are inherently self-short circuiting in nearby interconnected masses of metal. Voltage doesn't build up on a short-circuit.

This isn't what you are dealing with on this roof, if you can get a shock you are dealing with a potential difference relative to somewhere else. 

Eddy currents are analogous to the excitation caused by microwaving food, just in metal. They aren't that at all, but that's a good way to think about them.

I think you are dead right about the MPPT frequency playing a role, just the electrical mechanism isn't inductive, it's capacitive.

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9 hours ago, phil.g00 said:

Ah this is something different, still not eddy currents though.

 

9 hours ago, phil.g00 said:

I think your dealing with the capacitive effect of an electrostatic charge which does have plenty voltage, but normally not much current.

 

2 hours ago, phil.g00 said:

The thing is though eddy currents although they cause losses, they are inherently self-short circuiting in nearby interconnected masses of metal. Voltage doesn't build up on a short-circuit.

This isn't what you are dealing with on this roof, if you can get a shock you are dealing with a potential difference relative to somewhere else. 

Okay , so the shock is real and the term we use for it is not. Maybe the real description will be "Induced Voltage" is this case mistaken for "Induced Current caused by a change in a magnetic field. Actually both are caused by the change in magnetic field. 

At my previous work we had a 7 core cable between two substations almost 700 meters apart, used for monitoring different conditions and switching on one relay. because of the 220VAc over that distance in some of the cores created a "Ghost Voltage" in the Live and Neutral used for energizing the relay. It was enough to keep the relay energized after removing the live to its coil at the source. I had to put a nice resistor over its coil to ensure the relay de-energize when it should. I always said its caused by induced eddy currents in the cable, but now it seems like its actually two different concepts . If I could loop back the two end of that cable and connect the two ends of the "Neutral" core together, its a dead short and will that same Induced voltage now be seen as Eddy Currents? 

So if something is shorted the induced "Power" is called Eddy currents, but if its induced into a Conductor, its called Induced Voltage because it can "Build UP" to a potential difference? 

But both are caused by the Change in A Magnetic Field... 

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11 hours ago, Jaco de Jongh said:

My wife came and called me after she got shocked, i laughed at her and told her it is impossible, there was nothing that could shock her....... until I touched the mesh myself... guess who were the one laughing then? 

🤣🤣🤣 - This is so funny, because that is exactly what I said to my wife - until I got shocked🤣

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Thanks to all you guys for a very interesting discussion. My electrician put a couple of more earths on the panels, and he told me that resolved the problem. I didn't get a chance to test it after that, because I pulled my system apart for an upgrade. But as soon as my upgraded system is up and running again, I'll revert back here with my findings. 

All the best 

J

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On 2020/01/09 at 10:02 AM, jasonvanwyk said:

As I touched the ladder, I got an electrical shock.

Similar thing happened with me but it was the shower taps doing the shocking, this just after my solar installation. I did not realize it until my Wife and kids complained, and I said it’s impossible. I didn't really feel the shock. I think it’s electrician’s denial.

I than fitted more earth bonding straps to pipes and knew now it’s solved, that evening in the shower “shock” again.

I than knocked a earth spike close to the geyser and that evening “shock” again.

I kind of gave up on a solution, when asked by the kids I said I’m working on it!

Then few weeks later my surge protection order came, I installed it at the inverter JB together with another earth spike. Now that I read this forum I realized the shock disappeared and I don’t know what was the actual solution.

 

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3 hours ago, Jaco de Jongh said:

At my previous work we had a 7 core cable between two substations almost 700 meters apart, used for monitoring different conditions and switching on one relay. because of the 220VAc over that distance in some of the cores created a "Ghost Voltage" in the Live and Neutral used for energizing the relay. It was enough to keep the relay energized after removing the live to its coil at the source. I had to put a nice resistor over its coil to ensure the relay de-energize when it should.

I don't doubt it.

Voltages on "dead" cables could be there for a host of reasons.

A purely live cable draws a capacitive chargeing current,  at EHV level  they are sometimes just energised to act like a cap bank for the system. Most of the power system is inductive and the capacitive effect of a live cable can cancel some of this out. This charge will remain on the cable until it is discharged.

Once there is load current the unused cores will have a voltage because they act like a CT transformer winding if the current flow is assymetrical.

I suspect, in your example,  substations being very well-earthed, if your cable was armoured and glanded at both ends the sheath would conduct the current necessary to equalize the earth potential at both sub-station earths, so even if all cores were dead the transformer effect would be in play.  This current would be assymetrical, as it would behave as the single leg of a circuit.

A transformer has windings and a core. It is the windings that are live not the core, the core serves to concentrate the magnetic effect so that real power can be transferred between the windings though. The core is a magnetic circuit not an electrical one.

I differentiate between real electrical power transfer between the windings due to the transformer effect and the formation of eddy currents in the core. 

If you have noticed a transformer core is laminated, (split into plates), so in effect it isn't a solid lump of metal, it is many little cores.

This is done to precisely to limit the size of the available eddy current circuit in the core and increases the transformer's efficiency.

As eddy currents cause heating and are known as "iron losses" or "core losses" within the transformer. This will be represented by the current a transformer will draw when on not on load.

As opposed to "copper losses", which are caused by the impedance of the copper windings themselves once load current flows.

 

 

Edited by phil.g00
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13 hours ago, Jaco de Jongh said:

EDIT: 6 of my panels were lying flat on wire mesh supported 2.2meters from the ground, they were part of a string of 12 panels. the other 6 were on the roof, so only a + and - wire were running from the combiner box, through the air to the panels. No earth were installed. At midday, you could not stand on the ground barefoot and touch the wire mesh at the same time. Nice serious shock. There is no other power within 10 meters from that mesh. My wife came and called me after she got shocked, i laughed at her and told her it is impossible, there was nothing that could shock her....... until I touched the mesh myself... guess who were the one laughing then? 

I thought its static and tried to discharge it by touching it first and then hold on, I could not hold on much longer than a second and had to let go. Earthed the mesh, and no more shock. 

People die from static, it can be many kV.

I'd like to know more about this set up though.

When you say you earthed the mesh, how was it originally suspended -- on wood poles or something insulated from the ground?

And I take this was with an MPPT connected?

 

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