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Short (1ms) high current (~200A) current pulses - relay rating?

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Hi All,

I am building a surge / spike generator for a project to meet a rare std for which you currently cannot just buy a stuitably rated COTS device (if there was one, i'd buy it!)

The spike generator basically charges up a capacitor to a voltage (typically around 250V) and dumps that into the power supply lines of the DUT.  Overall energy per pulse is reasonably low, around 200mJ. but the pulses must be applied repetitively (1Hz repetition rate)

I want to put in a suitable output isolating relay / contactor to ensure HV safety, ie the devices Estop must be set, otherwise the output terminals are physicaly disconnected from the energy storage system, which necessarily uses semi-conductor switching to meet the timing requirements (pulse slope/ length etc).

So, to the question, does anyone have experinence of electro mechnical relays used to carry such a pulse, and how to take a guess at the rating for that relay?  Note the relay does NOT open or close under any current, it must have a suitable opening gap, and maybe be of a "force guided" multipole archtitecture to allow contact weld detection.

It's hard to know exactly what the relays contacts will experience during a 200A peak 1ms pulse?  The overal heating is pretty low, so my thoughts are to find a relay with large contacts, so thermally they can experience the brief resistive heating and not overheat?

ideally the relay will be PCB mount btw   :-+

i would use  igbt  parts instead   .... you're gonna burn the relay contacts sooner or later

There's plenty of large relays that can switch 200A. Most of them are low voltage, like 12V for car starters. The arc from connecting/disconnecting 200A at 250V would destroy the contacts. But since you don't need to make/break 200A you can probably overrate the relay voltage a lot.
A 250A 12V relay might work fine, assuming the insulation & clearance is ok

Finding a 250A relay that's PCB mount is going to be tricky though, all the relays I've seen that will do that current are chassie mount.  Any relay that large mounted to a PCB is probably going to need to be socketed, (eg socket is pcbmount and not the relay). And then there's the issue of making a socket that can handle 200A. Probably why its not usually done.  Chassie mount just makes more sense.

You could have multiple smaller PCB mount relays in parallel and share the load between them, since you're not switching active power with them you have time for all of them to fully connect/disconnect before a change in current flow. That makes it easier to parallel them safely and you could use 250 volt rated relays.   
If doing E-stop it might be a little tricky, since you can't stop a pulse that is already underway and have to wait for the pulse to be over before cutting the power. But it sounds like the pulses are so short that wouldn't be an issue.
If you go down this multiple relay route you'll probably need those large press-fit M6 or M8 through-hole ring terminal blocks on the PCB to get the 200A into and out of the PCB.

It's defiantly much easier if you can drop the PCB mount requirement . 

We used to pass 1500A through a motor reversing contactor with no issues as long as it was zero current switching, shouldn't be a problem for a suitably rated contactor.

I peak: 200 Amp assumed rectangular
Time On: 1 ms
Period:  1000 ms
Switch duty: Off Load only
Overall run time of the pulse string: not stated **
Circuit Voltage: not stated **

From my Resistance Welding days:Calc:
Dominant heating frequency is approx 1/ (2*1e-3) : ~ 500 Hz
Duty_Cycle= 1/1000 = 0.001

Equiv_RMS_I= (200^2)*0.001 = 6.3 Amp (6.3 Amp AC at 500 Hz) Edited, was 40A and was corrected by jbb
This may be too much practically for a pcb relay and pcb header etc

For small single or double pole relays, look at , for example, https://deltrol-controls.com/sites/default/files/pdf/data-sheet/900%20Series%20102011.pdf
The basic contacts there are UL listed for 30 Amp and motor start
There is also a 50 Amp contact set rated at 50 Amp thermal continuous.

** You would need to rfq the relay supplier with your above ratings, and overall run time of the test, and the circuit voltage.


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