Unusual welder question

[bluedog225]

I’m debating whether to run solar DC lines into the shop. Otherwise, I’ll leave all the DC outside and only bring in AC.

Assuming a worst case short from the solar supply to 2” rigid conduit at around 6500 watts, 400-500 volts DC, and say 17 amps. Give or take.

How long would it take to either catch adjacent wood on fire or burn through the rigid conduit in general, if ever?

Or would the conduit just dissipate that amount of heat?

Any guesses welcome.

 

[LopezBart]

A direct short will cause the output voltage to drop to near zero, the current may rise a bit. In the left hand diagram, you can see that the current output is nearly constant over a wide range of output voltages... solar panels are basically a current source. Note that a point point tracking controller will work to maximize the output power by letting selecting the load on the panel to produce most power (product of voltage and current); the controller then converts this (DC-DC converter) to charge a battery in the fastest manner.

 

[bluedog225]

I’m not sure how to interpret your post. Are you saying a direct short is not an issue?

What about faults that allow a sustained DC arc?

Thanks

 

[Sumboodie]

What's a welder have to do with this?

 

[bluedog225]

I figured welders would have a feel for the DC voltage and amperage needed to burn a hole in rigid conduit.

 

[LopezBart]

I’m not sure how to interpret your post. Are you saying a direct short is not an issue?

What about faults that allow a sustained DC arc?

Thanks

A direct short is not an issue; the delivered power would be zero. However, the NEC has some requirements, and since panels are outside, lightning protection is also an issue. I've seen high voltage fuses used; I'm not sure (other than lightning) what causes them to blow given the current source nature of the devices.

 

[bluedog225]

A class T would blow in an over current event but a sustained arc would, I think, be within the fuse capacity.

My purpose in asking the question is about fire safety. Having the lines in rigid conduit will help but I wonder about the case where the arc gets to burn away in there for a while. For example, a weekend or seasonally used place.

 

[Walkers]

A class T would blow in an over current event but a sustained arc would, I think, be within the fuse capacity.

My purpose in asking the question is about fire safety. Having the lines in rigid conduit will help but I wonder about the case where the arc gets to burn away in there for a while. For example, a weekend or seasonally used place.

Install the lines according to the code, have said lines inspected, notify you home owners insurance of the existence of the lines, use them for their intended purpose. That is it, don’t bring them in the shop for the winter, don’t weld with them, nothing. Just leave them alone in their conduit with their other friends and they will be happy.

 

[Beelzeboss]

Use PVC conduit?

 

[Doozer75]

A weldor is a man that welds.
A welder is a machine that welds.
Your post does not make sense.

-D

 

[bluedog225]

Rigid metal conduit.

 

[bluedog225]

Ok. Person who welds.

I’m was thinking people who weld would be all over this.

 

[PCustoms]

I’m was thinking people who weld would be all over this.

It's an electrical question, not a welder/weldor question.

Why are you running "6500 watts, 400-500 volts DC, and say 17 amps. Give or take." Without any circuit protection?

 

[PCustoms]

Also your math doesn't check out.

400vDC, 17A is 6800W

500vDC, 17A is 8500W.


Don't **** around with 400-500vDC, that's not something you're likely to walk away from.

 

[bluedog225]

I don’t weld so I may stumble around trying to get at this. My understanding is that solar panels produce direct current whether there is an arc fault or not. When there is an arc fault, a fuse won’t necessarily blow. They just buzz away till the arc is extinguished.

What if I asked it this way.

If you set a DC welder 400 volts DC and 17 amps, positive on an electrode and negative on the pipe. Would that burn a hole in schedule 40 pipe easily? Or would it take a while? Or is it not enough juice to do anything more than heat up the pipe.

Thanks

 

[PCustoms]

As far as I know, dc welding doesn't run at 400v, so again,that's not a welder question.



I'm not well versed in solar, but I'm not sure what your are talking about with "solar panels produce direct current whether there is an arc fault or not. When there is an arc fault, a fuse won’t necessarily blow".

Properly fused, a 400v short to ground will definitely blow a fuse/trip a breaker.

I was being serious earlier. Don't **** around with high voltage DC if you don't understand it. You won't survive.

Why are you running such a high voltage?

 

[bluedog225]

Thanks for the warning. 400 volts is not unusual for a solar array. Keeps the wire size to the controller manageable. Used with a combined box and rated disconnects.

With DC, there are series arc faults, parallel faults, and ground.

I recall code requires DC lines in the house to be in EMT. Seems too light to me.

I’m trying to get at is whether the energy involved with this array size (6500 watts) will burn through rigid conduit.

Whether it’s 375 volts and 17 amps or 220 volts and 30 amps (rough numbers).

Or is rigid going to be too heavy and have the heat dissipation capacity to handle an extended arc fault.

And I’m curious what voltages do DC welders run at?

Hope this makes sense.

And thanks

 

[Just_Steve]

Thanks for the warning. 400 volts is not unusual for a solar array. Keeps the wire size to the controller manageable. Used with a combined box and rated disconnects.

With DC, there are series arc faults, parallel faults, and ground.

I recall code requires DC lines in the house to be in EMT. Seems too light to me.

I’m trying to get at is whether the energy involved with this array size (6500 watts) will burn through rigid conduit.

Whether it’s 375 volts and 17 amps or 220 volts and 30 amps (rough numbers).

Or is rigid going to be too heavy and have the heat dissipation capacity to handle an extended arc fault.

And I’m curious what voltages do DC welders run at?

Hope this makes sense.

And thanks

Go with micro inverters instead of a string inverter and keep all of the DC voltage on the roof and only AC voltage coming into the house. I have 41 panels and 41 micro inverters.

 

[bluedog225]

I’d love some micro inverters. Those aren’t in the plan for this summer’s deployment. Maybe later later. Keeping all the DC on the roof would be great.

I’m hoping there are some people on this forum who have some experience melting metal with electricity who might be willing to chime in.

As I indicated above, educated guesses are welcome.

 

[bluedog225]

duplicate deleted

 

[rlitman]

It's an electrical question, not a welder/weldor question.

Why are you running "6500 watts, 400-500 volts DC, and say 17 amps. Give or take." Without any circuit protection?

I work with 540VDC battery banks that have 300A circuit breakers on each string, and coffee can sized fuses on the DC bus. In my situation, I'm confident that something would clear a fault before the EMT was damaged. But that's because the short circuit current easily exceeds the magnetic trip threshold of the breakers.

At less than 20A coming off the panels, you have a very odd situation where the short circuit current may very well be too little to trip a thermal-magnetic breaker, so you could end up with a smoldering situation. 6.5kW is easily enough energy to burn through even rigid conduit. I'm not a PV expert, and wasn't familiar with DC AFCI technology, but it seems that systems over 80V need it.

PV Arc Fault Circuit Interrupter | SMA America

www.sma-america.com

"The listing standard for certification of PV AFCI devices is UL Subject 1699B, Photovoltaic (PV) DC Arc-Fault Circuit Protection, which requires PV AFCI devices to behave according to the requirements of 2011 NEC Section 690.11. UL Subject 1699B stipulates that a PV AFCI device must detect an electric arc of 300 W or more, and interrupt it within a maximum time period of two seconds."

And yeah, that settles it, don't bring the DC inside!

 

[bluedog225]

Thanks @rlitman !

That’s what I needed.

It makes too much sense to put the inverter out in the conex and only bring in traditional AC into the building.

The DC arc fault stuff is all over the map in the solar area. Apparently detection and panel level shutdown is the answer but the tech still seems a little sketchy. SMA is probably one of the best.

Don’t quote me on this but I believe a DC parallel fault may even decrease the amperage. Making the circuit protection more difficult.

 

[Just_Steve]

I work with 540VDC battery banks that have 300A circuit breakers on each string, and coffee can sized fuses on the DC bus. In my situation, I'm confident that something would clear a fault before the EMT was damaged. But that's because the short circuit current easily exceeds the magnetic trip threshold of the breakers.

At less than 20A coming off the panels, you have a very odd situation where the short circuit current may very well be too little to trip a thermal-magnetic breaker, so you could end up with a smoldering situation. 6.5kW is easily enough energy to burn through even rigid conduit. I'm not a PV expert, and wasn't familiar with DC AFCI technology, but it seems that systems over 80V need it.

PV Arc Fault Circuit Interrupter | SMA America

www.sma-america.com

"The listing standard for certification of PV AFCI devices is UL Subject 1699B, Photovoltaic (PV) DC Arc-Fault Circuit Protection, which requires PV AFCI devices to behave according to the requirements of 2011 NEC Section 690.11. UL Subject 1699B stipulates that a PV AFCI device must detect an electric arc of 300 W or more, and interrupt it within a maximum time period of two seconds."

And yeah, that settles it, don't bring the DC inside!

Great answer and the link was very informative, I learned something new today.

 

[cannuck]

Nobody replied to the OP's question about welding voltage. Most DC welders work around 40 volts. What isn't clear to me is if your battery bank is in series with the line you want come inside with. If it is, I suggest you take an old wrench and dead short it between the posts of a 12V car battery to realize that heating happens EXTREMELY fast and your wrench will be bright red in seconds.

I worked as the mechanical guy around HV electrics, and THE most dangerous voltages are 480 and 600 - because they are all over the place down low where devices are connected. Unlike 208 or 240, 480 and 600 will hurt you badly. I remember an engineering technologist doing an inspection on a distribution who accidentally touched a live 600 terminal. The reaction in his body was so strong the jerk from his muscles broke his leg. Fortunately the path went down his side to the leg/shoe at ground as crossing his chest cavity probably would have killed him. Once you get to the really big voltages, the safety precautions are huge and everything is way up high and I can't think of a single incident over the last 35 years that hurt or even threatened anyone from transmission voltages.

 

[rlitman]

Nobody replied to the OP's question about welding voltage. Most DC welders work around 40 volts. What isn't clear to me is if your battery bank is in series with the line you want come inside with. If it is, I suggest you take an old wrench and dead short it between the posts of a 12V car battery to realize that heating happens EXTREMELY fast and your wrench will be bright red in seconds...

As I said in my post, batteries can easily be protected by a fuse or magnetic circuit breaker, because of the available short circuit current. But the OP is talking about a Photo-Voltaic array. It behaves VERY differently than a battery, because solar panels are not energy storage devices, so we cannot treat them like batteries. PV short circuits behave in a unique way that doesn't mesh well with conventional means of protection developed around batteries and utility power.

As for welding voltage, that depends a lot on the process, but most arc welding happens between 15V and 33V, though OCV (when you're not welding) can go much higher (up to around 80V works well), particularly with stick processes.

 

[bluedog225]

Thanks for the welding info.

This would be a line coming straight from the solar panel array into the charge controller. DC. For example, sixteen 300 W panels with an open circuit voltage of somewhere around 40 V and about 9 A.

Eight panels and series and the two strings paralleled would give me About 320 V and 18 A.

Generally, I want to run less than 30 A on the PV line to keep my wire at 10 AWG.

The chance of something going wrong and creating a fault inside the rigid metal conduit is pretty remote. But still, with no way to protect against it I’d rather not put the structure at risk.

The high voltage has my respect and I plan on proceeding with extreme caution. I’ll only hook up the panels on a dark cloudy night and into a combiner box with appropriately rated disconnects at the solar array and again at the controller.

The battery bank is 48 V. And it has an appropriately sized class T fuse from blue sea.

 

[rlitman]

...The chance of something going wrong and creating a fault inside the rigid metal conduit is pretty remote. But still, with no way to protect against it I’d rather not put the structure at risk...

Probably. Unless the wire in the conduit was damaged during installation, your faults usually happen at connection points. And PV systems see a lot of "weather", so do keep that in mind.

 

[cannuck]

As I said in my post, batteries can easily be protected by a fuse or magnetic circuit breaker, because of the available short circuit current. But the OP is talking about a Photo-Voltaic array. It behaves VERY differently than a battery, because solar panels are not energy storage devices, so we

You stated that it is easy to current limit such a circuit, but we have no idea if the OP had or was about to do so. I have seen a few DIY systems and even a couple of old glass jar standby systems where there was a fair run of full voltage and unprotected long runs in the installations.