The drop through mine is negligible (20A - 200A). If they are not fused, there is no drop to speak of. If they are fused, the drop is related to the type of fuse.

 

AutoTech, you shouldn't have any voltage drop. If you are experiencing any it is because the contacts have excessive wear due to operating (disconnecting) under load or loose connections. How or why are you asking this question, is it just for info or are you indeed having a measured drop in voltage? If it is a measured voltage drop, did you take a voltage reading at the source first, and then at the load side of the disconnect? Bob

 

I've been having problems with (a) the utility company --line voltages all over the map and (b) length of runs into my shop which has affected some of the more sensitive (inverter-based) equipment. It's all additive, that's why I needed help on the disconnects.

 

Size and type of conductors is more important than length of run. Are they above ground or buried, installed by an licensed electrical contractor or by others? Sometimes when we think the problems that are happening are caused by the power company are not the case. The problems could be coming from our connections at the service/meter pan or breaker contacts to the bus bars or corrosion at the terminals on the breakers or a breaker that is defective. Just a few of the problems that I have found from customers who have had your same complaints. It is my recommendation that you have an licensed electrical contractor evaluate your problems and then go with his suggestions. Just my experience. Bob

 

Burial and yes they were installed by a licensed electrical contractor (who is very good --- they did my residence as well and their work is exceptional). The problem is, has been the pain in the butt utility (I'm in a remote area and use a "electrical cooperative" (farm type)........................

 

Have your electrical connections been retorqued since the initial install? I see this with a lot of equipment, (especially new installs one year later) and is an annual maintenance requirement to check for corrosion and torque values.

 

There is always some voltage drop at any connection at any set of points. There is always some air between contacts. Or some voids with air in them. Air transfers electricity faster than metal. So the air actually starts to raise the voltage of the surface of the receiving contact before the metal can conduct it. Causing a slight resistance to current flow. It is very slight though, in a manual throw disconnect. Probably millivolts even at high amperage.

If you take temperature readings of equipment you will see any contact has a higher temperature than the cables or bars that feed them.

If you wonder about the tiny air gap or thin film of oxidized copper, consider this product below. Surely there is going to be a very thin layer of that grease between the contacts. On older welders you might need a contact grease to allow switching of heavy current switches, that grease is supposed to carry current and create a very small resistance.

https://www.amazon.com/gp/product/B00HDF9EXE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

This is a tricky subject though. You can get burned either way.

Sincerely,

William McCormick

 

...If you wonder about the tiny air gap or thin film of oxidized copper, consider this product below. Surely there is going to be a very thin layer of that grease between the contacts. On older welders you might need a contact grease to allow switching of heavy current switches, that grease is supposed to carry current and create a very small resistance.

https://www.amazon.com/gp/product/B00HDF9EXE/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

That's an option, but better contacts are not pure copper. Silver oxide has the same resistance as silver (unlike copper oxide, which is a semi-conductor). That is why silver plating is used on more critical parts, and why transfer switch contacts for example are silver alloy (pure silver has the issue of welding itself, so alloys are used to avoid that).

Anyway, if you want to avoid the oxide issue altogether, you can silver plate parts if they are plain copper. This is what I use:
http://www.cool-amp.com/cool_amp.html
But the plating from this is too thin to last on contacts. I use it on bus bars, and on fuses.

But again, if you're looking at a horsepower rated kill switch, you can be pretty sure the contacts are already silver alloy, and are already lubricated.

 

If you get a 23 millivolt drop on a 240 volt circuit at a set of contacts, that is about five watts of heat if your service is supplying 100 amps. That is substantial.


Sincerely,

William McCormick

 

That is why in the fire alarm business, in NYC you cannot splice wires you have to make a continuous run for risers and to sub panels. Although after fires or accidental damage from construction it is done. It is not allowed on original submissions.


Sincerely,

William McCormick

 

Lots of info here about voltage drop and contacts. However, if one has any doubts about whether or not they have voltage drop across the contacts of a disconnect, using an accurate voltmeter, put a non-varying load on the circuit, take a measurement across the line side of the disconnect and then across the load side. I doubt you will see any difference as long as the contacts are in good condition. We are not talking rocket science here. Bob

 

The drop is small and you will see it on a millivolt scale when drawing high amps.

Look at at Ethernet connections you cannot have too many on a single run, or you will start to lose communication noticeably, and those contacts are designed for minimal loss sometimes made of gold.

Just take a look at contacts through a thermal camera, there can be no doubt that the contact is a weak resistive link. Look at the size, the square inches that are needed for light pressure high amperage plug contacts. Thermal cameras will change anyones opinion about contacts.

Having replaced many a welding plug in my day for friends and family, I can assure you that the heat is at the plug not in the wire if the wire is even close to being the right size.

Sincerely,

William McCormick

 

Once our local phone company looked at grounding, how could we improve its resistance to lightning damage. The most significant concept they came up with was corner radius. High voltage/high current charges aren't receptive to sharp turns. The loss where electrons must make sharp turns is significant. Phone company engineers paid us handsomely to replace grounding conductors without any sharp bends I wonder in many connectors, or relays how substantial this phenomenon might be?

They also came up with terminals that had two bolt holes to bolt to the buss bar, and two set screws to clamp to the conductor. They also insisted on similar metals; brass, bronze, copper.

Willie

 

Gentlemen, the question started about voltage drop across the contacts of a heavy duty disconnect switch and the load side feeding welding stations. Again, this is not rocket science! We who are in the electrical trade of course know there is most always voltage drop across a set of contacts but for this application, it is not going to amount to any noticeable difference. If this was such a big problem, then the manufacturers of this equipment and UL inspectors would be getting involved with this issue. Bob

 

I think you are right about good quality equipment built for the purpose. Old salvaged equipment, or very cheap imported stuff that may not be the case. In any event, I see far more loss at terminations than at contact points. I'm a big believer in clean, shiny terminals, and conductors matched according to size ranges intended, I use Noalox on both aluminum and copper, and use a calibrated torque wrench.

Willie

 

One other problem with a disconnect switch and an induction load is that upon opening a shorted or out of control situation, perhaps where the welder or weld transformer is charging and discharging the often rather large transformer, your manual disconnect could experience catastrophic failure.

My opinion about this before a local accident was that the switch would be a great idea. Then a friend at a local supply shop who had heard the rooftop AC unit on his building banging, (unit was starting and stoping making a racket) ran to the switch gear and pulled the handle on the disconnect for that unit. He said as he pulled the handle, to a box he had never seen the inside of before, that he actually saw the internal parts of the switch. And then his face and arms burned off. He went for years of reconstructive surgery. As far as I know there was only 200 amps available to that switch and I believe there was a lower amperage breaker between that 200 amp breaker and the disconnect.

After this accident I recalled older electricians and firemen explaining that manual disconnects are only good to turn things on, or keep them off. This was in reference to large shears and brakes/breaks. After the AC disconnect accident I got what they meant. They also said if you have to throw it throw it hard and fast at arms length. I thought it an exaggeration until my friend burned his face off.

My friend at a motor rewinding shop always recommended a motor start relay, or spring actuated manual disconnect. So that the user is not responsible for the speed of disconnect.

My old boss an electrician and an ex-safety officer from a local fire department used to put his hand between his face and the panel box when throwing a breaker for the first time. I do the same now. He had or was witness to a similar experience while energizing a breaker.

As welders I guess this should be second nature to some extent.

Sincerely,

William McCormick

 

If you guys have ever ARC welded with a large P&H or other similar old TIG and ARC machine. You know that you get instant start, you get an amazing ARC that is extremely hot and penetrating even at low amperage. The reason is because those machines maintain a certain voltage, almost regardless of the ARC length or amperage. I often weld with a quarter of the amperage of other gasoline driven welders, and other smaller stick only welders.

Imagine now you have a failed welder, the transformer is connecting and disconnecting, the fallout voltage of a transformer can easily reach 10,000 volts probably reaching 30,000 volts, imagine the welding that is capable of. That in my opinion is why the wrong disconnect can be worse than none at all.

Sincerely,

William McCormick