Off Topic - DC Safety

Steve Etter

Lifetime Supporting Member + Moderator
Join Date
Apr 2002
Location
Morristown, TN
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Alright....another safety issue......

With the improvements we have all seen with the implementation of Ground Fault Interupts for AC circuits, one thing I haven't really seen is how one should create a GFI circuit for DC circuits.

The specific application I have is a machine in a "wet" enviroment where the manufacturer is recommending 1/2 HP, 90VDC motors on the conveyors. Because there is a real safety concern in the environment, how should the DC side of the system be protected?

Also, since the DC system has variable speed from 0 to 100%, one of our guys has also brought up the question of whether any GFI system would be effective down in the lower speeds since armature current and voltage can be very low.

Yes, for those of you who wonder, the AC side of the machine is already being designed with GFI protection.

Steve
 
Steve

The ground fault detection for DC systems I have come across in the past were in the form of a current operated relay. When a DC earth fault occurred current would have to flow through the coil of the relay back to it's source, then if the current was high enough the relay would operate and shut off power.

I will admit, this form of protection was on a British rail locomotive built in the early 60's, but it is the nearest to what you want that I could think of.

Maybe you could use some DC shunts in the motor circuit, one on the 'positive' side and one on the 'negative' side, and then compare the volt drop across the shunts for a difference. :confused:

Anyone else any ideas??

Paul
 
The basic operation of an AC GFI (or RCD) should be the same for a DC version. As Paul stated, you just want to measure the current imbalance, and turn off the circuit if it exceeds some preset value. The lower voltage/current at lower speeds shouldn't matter. As long as the currents can be measured, they can be compared.

A quick Google search didn't find any off-the-shelf DC GFIs, but I found some references to them at sites about solar power.

beerchug

-Eric
 
Yeah, I also did a Google search and did find some devices, but having absolutely no reference to use, I have doubts about what I should be looking for.

My real hope is to hear from someone who has experience with DC GFIs and hopefully some engineering data or references.

I am somewhat surprised how there seems to be so little experience with this (my own included).

I want to find a device or technique that would be Industry Approved rather than trying to cobble something together myself.

Steve
 
I put a couple on an Electro-Motive locomotive in 87 or 88 for a test. Don't remember any particulars, but will see If I can locate someone who hasn't been "right-sized" who might know. They weren't very expensive, were a module, perhaps 5" x 5".

Never saw them actually work, but similar to ac version.

regards.....casey
 
DC Ground fault protection

Only major experience I had with DC is on board nuclear sub. 250 volt DC battery distribution etc. Like the (AC 3 ph 450) it was ungrounded. Tolerance for resistance to ground was 50 K. Battery voltage could vary between around 190 (full discharge) up to 320 or so (charging voltage). Most of the time low resistance to ground meant we had to clean the battery.

Unless the thinking has changed the guidelines for electric shock are 30 volt and 10 milliamps )IF I recall correctly).

IF 90 VDC is ungrounded then an occasional check with VOM would suffice I think.
Could install a ground detector at the power supply ??

Is the 90 VDC grounded on one side?
IF grounded then ground fault protection may be needed.

Two shunts (one in each line) could be compared for the difference in line amps similar to that done in GFCI.
ASSUMING amps are max 30
then line 1 at 30 minus line 2 at 29.99 = .01 (10 mA)
ON a 30 amp shunt 30 amp = 50 mVolt.
and there is .6 mV per line amp
so a 10 mA difference would equate to a volt difference of 5 microvolt.

Dan Bentler
 
Hey, they were BENDER!

And it was 97 or 98, not ten years earlier.

I was able to get a hold of someone who said they did wind up working quite well.

Problem with DC locomotives is that moisture builds up in the traction motors (1000 hp). The control cabinet blower is usually route to the traction motors for cooling, cuasing moisture, shorting out the 2000vdc at 2000 amps, tripping out protective relays in the controls and locking out a locomotive, bad if you are on a hill or making a panic stop.

GFI relays sensed for a "REAL" problem, to allow loco to keep operating.

Nifty idea, but a few hundred dollars per unit was TOO much money.

Now, when someone gets killed, will it be TOO much $$$.

regards.....casey
 
Yeah,,, I know I goofed! but at least it is the right site to go to... last URL I posted was a dead end!


ps.
Well Rick and I had good results! and if your wondering I did read all the post... just missed the link Rick posted,,, hmmmm I got to many grey hairs!
 
Last edited:
Bitmore said:
Hey Casey how ya doin' with you to do list?

Things To Do:

Hang out on plcs.net
done.gif

Try to remember the meaning of life
Oh yeah, Find a great job and make a huge salary

One down... Two to go... (y)

beerchug

-Eric
 
Things To Do:

Things To Do:
Hang out on plcs.net
Try to remember the meaning of life
Oh yeah, Find a great job and make a huge salary


Well, do hang out on plcs.net a lot.

Don't remember why I wanted to remember the meaning of life.

Had some interesting conversations on plc's, engine controls, switchgear, and ridiculous $ this week. I don't need ridiculous $, just steady $. Just hope that I won't have to try to incorporate OMRON plc's into generator sets. GE and Entertron would be just fine!

regards.....casey



Remember, why use a ridiculous word, when an incidious one will do just fine!



As I look back, some of the worst shocks I got were DC.

@4 volt cranking batteries on a 16 cylinder CAT, was pulling batterry cables off. Left hand on one terminal, right thumb (with a metal sliver in it) on the other post. OUCH.

Then there was the time I was tuning a shortwave receiver, and my arm brushed across the top of the telegraph key (960 volts for the screen voltage, key up). Still have a nice red mark on my arm (RF burn) 34 years later.
 
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