current limiting 1

[sparky970]

I am having an issue with measuring voltage across an electrolyser cell in a Chlor-Alkali plant. We are just finishing start-up/performance tests and need to resolve this problem. First of all, we have 66 cells with a system voltage of 300V and 100KA which will allow us to have a voltage drop of 4.5V-5V across each cell. The circuit we are using to measure the voltage consists of a fuse and a 10K ohm resistor in series for both the positive and negative side, this is used as an input to an E/I. We are seeing a .1V-.3V voltage drop on the 10K resistor depending on the load, this produces an inaccurate measurment of actual voltage drop across the cell. Are these current limiting resistors absolutely necessary? Is there another way to measure this more accurately? Any help would be appreciated.

 

[Skogsgurra]

Having hundreds of volts and 100 kA available makes any measurement dangerous. So the 10 kohms (I hope you have one resistor in the plus AND the minus lead!) are necessary to avoid that "Big Current Happen" as the Japanese guys put it.

Ten or twenty kohms should not cause such a high voltage drop if you use a modern DMM. They usually have a 10 Mohms internal impedance, so you should only lose about 0.2 percent, i.e, 10 mV when you measure 5 V.

Of course, if you are lucky enough to have a Simpson or some other goodie, you can have that voltage drop - easily. Then I suggest that you get fused probes instead. They are available with different fuse ratings and have an internal resistance that is orders of magnitude lower.

But do not work without TWO current limiting probes - or fused probes!

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

You could consider using an isolation amplifier with fused connections to the bus. The isolation amp provides the output signal to your instrument. The fuses should really be capable of breaking the prospective fault current, which on a system rated for 100kA is going to be stupendous. In practice the chances of achieving that fault level from only 300V is extremely remote, but the fault would still be big enough to cause you to have a Very Bad Day if you do not have either a resistor to limit the current, or a fuse to break it. The resistor is probably a cheaper solution to providing a fuse of adequate breaking capacity.


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Sometimes I only open my mouth to swap feet...

 

[Skogsgurra]

I use these.

http://www.obiat.com.au/silvertronic_p1.htm

They have a 500 mA fuse and there seems to be a low ohm resistor built in to reduce short-circuit current. They are good for 600 V and 50 kA prospective SC current. So I think they will do a good job in your application. As Scotty says, you will probably not get as much current as 100 kA, those rectifiers are usually not grounded.

Gunnar Englund

http://www.gke.org

 

[ScottyUK]

One other possibility is that the resistor is there to prevent someone connecting a grounded instrument such as a normal bench 'scope directly to the DC bus, thus introducing one ground fault. A second ground fault on the other pole of the DC bus, either present before the connection of the instrument or occuring while it was connected, would have a bad (i.e. terminal) effect on the instrument and potentially the user too.


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Sometimes I only open my mouth to swap feet...