Use of grounding straps on a 250 vdc ungrounded system

[steely605]

At our plant, our crane repair personnel require the electrical department to put up grounding straps by shorting our dc crane rails and connecting to ground. Our system is 250 vdc ungrounded. Crane repair is led to believe that these grounding straps will protect them if the power is put back while they are working by the hotrails. We use proper lock out tag out procedures. We recently had a instance where the power was put in and the ground straps were not removed. The straps burned for at least 5 minutes and blew open. Our thermal magnetic breaker never tripped. In my 35 years experience, I have seen this happen at least ten times. We have been trying to convince our company to get rid of this practice of grounding straps. These strps are the the largest straps we can buy from Sailisbury (4/0) and they are not rated for DC. Now they are talking about running 1000 kcmil cable to a disconnect with a Kirk-key application and making that disconnect into a deliberate short circuit if the power goes in. They base their criteria on principles of grounding straps used by high voltage workers on AC circuits. I am saying that we are creating more danger by using th grounding straps that what we are trying to prevent. We are saying that lock out and tag out is that best and only measure that should be used. I would like to here some expert opinion on this subject. Am I off base here by disagreeing with these grounding straps and a so called short cicuit disconnect they are thinking about installing?

 

[dbuckley]

Well, if its happened at least ten times and its the electrical department that put the ground straps on and take them off, then the processes that manage this are clearly not working, so I think your crane workers are right to worry.

 

[dereckbc]

OK I got a couple of questions and comments, but not sure I understand.

You state you connect each rail, which I assume is the positive and negative, to ground. Please define ground?

Are you talking to earth or the frame of the crane. If it is the frame of the crane then you are shorting out the positive and negative buss, which would offer protection. IT would be the same as tying or shorting the buss with cable.

Next you stated:

These straps are the largest straps we can buy from Sailisbury (4/0) and they are not rated for DC.

Well it doesn’t matter if it is AC or DC. Straps or cable made of copper (or insert any metal you want here) don’t care if it is AC or DC. In fact DC would be a lower resistance because there is no inductive reactance to deal with.

 

[zbang]

Seems to me that there are at least two problems. First is that protection procedures (LOTO) don't include the properly application -and- removal of the jumper, so the jumper probably shouldn't be placed at all. And second that there isn't a continuity check to see if the jumper has been removed (depending on the equipment, this may not be practical, anyway). Or, you could look at it that the reenergizing procedure is lacking some steps.

OTOH, while a jumper will keep the rail-to-rail potential lower than 250v (I don't see how connecting to "ground" helps much), I've seen the meter on a 6000 amp DC crane supply peg when lifting a load (450 ton crane in a steel mill). You need a pretty large strap to trip that OC device.

 

[quogueelectric]

Put some sort of ground detection on your feeder buss this will satisfy all involved.

 

[steely605]

grounding straps

grounding straps

dereckbc,

The ground is placed to the building column, earth ground. Grounding doesn't make sense since our dc system isn't grounded. I understand the lower resistance value of the dc in the 4/0 cable but our problem is that the thermal magnetic breakers do not trip. I would expect this breaker to trip on a dead short.

Would you say that if this breaker doesn't trip, that we are putting objectionable current on our AC system grounding path? Would this be a violation of 250.6 in the NEC?

This is my first time posting and I find everyone's comments very helpful.

 

[dereckbc]

dereckbc,

The ground is placed to the building column, earth ground. Grounding doesn't make sense since our dc system isn't grounded.

Are both positive and negative rails grounded or just one? I understand the system is not grounded, but the reason for that is in the event one of the busses is ground during normal operation, it will not have any effect unless both polarities are grounded. It works just like a delta or any other ungrounded system. The point your safety people are trying to make is for maintenance procedures not operations

I understand the lower resistance value of the dc in the 4/0 cable but our problem is that the thermal magnetic breakers do not trip. I would expect this breaker to trip on a dead short.

I don't understand that either and is why I asked if both rails are being grounded and to what? Are the columns bonded together electrically, or the jumpers going to the same point physically? What I suspect is you ground each rail to a different column and the resistance is just high enough between the two points that the breaker only sees it as a normal load current. I don’t know much about cranes but I suspect the motor draws a large amount of current and your jumpers combined with being bonded to different points just do not have low enough resistance to operate the breaker. What size is the breaker, and what is the DC supply current rated at?

A better way may be to bond the rails together with one jumper made of something like 750 MCM, and then bond one rail to ground with another single jumper. Now that I think about it, the ground jumper may not be needed, but it wouldn’t hurt anything. Since the system is not grounded, you should be able to touch either rail under normal operation and nothing should happen. It is only if you come into contact with both rails you would be electrocuted. As mentioned earlier I would have a ground fault detection system installed, like you would an ungrounded AC system.

Would you say that if this breaker doesn't trip, that we are putting objectionable current on our AC system grounding path? Would this be a violation of 250.6 in the NEC?

No. Because it is not normal operating conditions, it is maintenance.