# need help switching large current

#### elimenohpee

Joined Oct 26, 2008
47
What do you recommend for switching a large amount of current? The amount of current I'm testing on is around 120A. I'm simulating worst case senarios at work, and I need some kind of switch to connect them together, as I don't want physically touch the wires lol. I was thinking a Frankenstein knife switch, but I can't seem to find any at Lowes/Home Depot rated for high amounts of current.

Another thing I was thinking was using a AC disconnect system. However the test I'm conducting is using a 24VDC system. Would this allow me room to use a lower rated system? I'm thinking in terms of power generation instead of the current rating. A lot of things are rated for 60A which equates to P=(120)(60)=7.2kW, but in my case P=(24)(120)=2.88kW. Is that a safe way to think of the heating element?

Any ideas?

Last edited:

#### jpanhalt

Joined Jan 18, 2008
11,087
Are you wanting to switch AC or DC? Is the load inductive or purely resistive? What voltage?

Battery disconnects for trucks have ratings into the several hundred amp range, but it is DC. Don't know if they have AC ratings too.

John

#### elimenohpee

Joined Oct 26, 2008
47
Its switching 24V DC and a purely resistive load, although there might be slight traces of natural inductance in the system. I thought about using a safety switch, I found one laying around the shop thats rated for 60A @120VAC. What I'm thinking though, is if the voltage is lowered to 24V DC, then I can afford to pump more current through at that lower voltage, as long as the provided current is less than the power ratings of the switch for AC.

Do you think I am safe in my assumption?

#### jpanhalt

Joined Jan 18, 2008
11,087
You cannot make that assumption. Switch ratings for AC and DC are different.

I would consider either a shut off switch as mentioned or a starter relay. For our high-current DC winches, we use two starter relays in series for added safety. That way, if one freezes closed, the other offers backup. As an added safety feature, we add an LED to each to ensure both are working. Otherwise, one could stop working and you wouldn't know it until the other failed too.

A 120-A-rated starting relay is nothing.

John

#### mcgyvr

Joined Oct 15, 2009
5,394
I'd use a circuit breaker rated for your DC (probably find them with 65 or 80VDC max rating) and 150A trip rating. Thats what I use here in our lab for applying loads during testing... I actually go up to 600A at 48VDC so I simply have some 800A 80vdc DC breakers. Big suckers.

#### BillB3857

Joined Feb 28, 2009
2,569
Another question not yet asked is, "Are you using this device to apply power or remove power from the load?" It makes a difference. Think of a fuse. They have a rating for 'interrupting" capacity, not connecting capacity.

Joined Jun 1, 2009
499
Breakers are meant to be safety measures, if they trip something is wrong, fix it, reset the breaker under no-load conditions and try again. If the breaker trips and you do nothing and simply flip the breaker back under a load near it's trip limit or a dead short there's a chance that the arc from the contact bounce will weld the contacts and then things start to blow up. I don't personally think this chance is very high, but breakers were NOT made to switch loads.

#### mcgyvr

Joined Oct 15, 2009
5,394
breakers... "break" and make loads all the time and are designed and tested for interruption of full load currents for many..many cycles. Most DC circuit breakers are typically also UL listed for use as switches do to the increased construction over AC breakers. I would not recommend their use for multipe repetetive make-break cycles but for use in a testing environment to energize-deenergize full loads a few times a week or month will not be an issue. Plus the added benefit of a safety device makes it perfect for this application.

#### Jaguarjoe

Joined Apr 7, 2010
767

#### BillB3857

Joined Feb 28, 2009
2,569
OR... use two of the units from post #9 with the coils in series and that will give you the 24V control you need PLUS it will give you the series redundancy in case one welds closed, as mentioned earlier.

#### kubeek

Joined Sep 20, 2005
5,791
OR... use two of the units from post #9 with the coils in series and that will give you the 24V control you need PLUS it will give you the series redundancy in case one welds closed, as mentioned earlier.
I don´t think that two 12V rated switches in series will give you 24V capability. One will allways switch a little sooner then the other, so it will be effectively switching 24V anyway.

#### jpanhalt

Joined Jan 18, 2008
11,087

#### BillB3857

Joined Feb 28, 2009
2,569
I don´t think that two 12V rated switches in series will give you 24V capability. One will allways switch a little sooner then the other, so it will be effectively switching 24V anyway.

I was focusing on the 12 volt coils in series but since the starter relays are designed for an inductive load (motor) and the OP is using a resistive load, there shouldn't be much of a problem interrupting the current. Resistive load won't give the inductive kick to start an arc. Others may think differently and should join the discussion.

#### timrobbins

Joined Aug 29, 2009
318
You have not identified the power source characteristic - is it a battery (ie. much higher prospective current), or a power supply (ie. current limited)?

But since you are in a test environment (ie. in a controlled, restricted environment), and you have a fairly constant resistance (ie. not an incandescent bulb), then I suggest just using a three phase AC isolation switch, or cb, or contactor, with all poles in parallel, with nominal 50A pole rating - that would be a small DIN type cb found at an electrical outlet (and often have a DC rating, but I don't think that really matters in your test environment).

If you had a professional setup then you would use something like an Albright SW64 or SW80 contactor.

Ciao, Tim

#### retched

Joined Dec 5, 2009
5,208
We use fused knife switches for such currents. -- Fun to watch. And they make you want to say "ITS ALIVE!!" when you throw them. Rubber and fiberglass coated wooden handles. NICE.

For automated or assisted, the automotive starter relay(golf cart solenoid) is on our list. We have many in our shop due to having 60+ golf carts on campus.

For the HVAC applications, the good ole contactors have been serving us for many years. And, they seem to act up for awhile before failing, so you have some notice of failure if not using redundancy

Knife switches rated to 6000 amperes
http://knifeswitch.com/