87CR250R
Member
- Location
- Oakland, CA
I work a a maintenance mechanic and maintain a fleet of ship docking tug boats. Most of the electrical work I deal with is troubleshooting motor controllers. I have had several failures recently that have brought to light what I believe to be a potential safety problem. In summary, our 480v generation and distribution system on the boat is set up for Delta configuration. As I have found out recently, our shore power transformer is 480v Wye. I think this is a problem but I am having a hard time convincing the company that this is a safety issue.
For a more detailed description, each boat has two 150-175kw generators wired 480v Delta. The generator chassis is bonded to the hull and we take 3 leads from each into the switchboard. In the switchboard all of the separate chassis have grounding wires that tie them into a common terminal strip which is bonded to the hull. There are 3 small control transformers wired into Wye configuration to create a neutral tied to ground. These form our ground indicating circuit for the 480v system. This is all pretty standard. There is a single 45kva 208Y/120 transformer for lighting loads. This system bonds the neutral to same common ground terminal strip one half of the boats in the class and to a separate on the other half. There is a current transducer on the neutral to ground connection for a ground fault indication.
Everything I have described about the distribution system is standard. The lighting systems in the past have typically been Delta but modern boats have been using Wye more and more frequently. So the issue I have is once the boats are tied into the Wye shore power. The shore power is a 4 conductor cable, 3 hot and a ground that connects to the switchboard. The ground from the shore power is tied into the same terminal strip as the 480v Delta grounds. The first issue I have seen is that if you do have a ground fault on shore power, this connection has rendered the 480v ground indicating lights inoperative (they all stay lit). I'm not sure why this is, yet. But the biggest issue I see is that I have had ground faults in equipment that cause the equipment to energize at 277v through the ground connection to the neutral in the shore power transformer. I first experienced this on a hawser winch with a 480/240v brake solenoid. A screw in a junction box had pierced the insulation on a wire. The winch failed to operate on generator power because the brake would not release and the motor could not overcome the brake. The boat comes in for me to troubleshoot, we plug in, and everything works normally. The screw pierced the wire in such a way that power from L1 could flow to ground through one half of the solenoid (which effectively rewired it for 240v with half the power available). So, on shore power, L1 to ground generated 277v which was enough to drive the solenoid.
More recently on an older boat, I had a similar problem with a chaffed wire in a pressure switch controlling an air compressor. Upon energizing the controller (with the switch in the off position), the controller began buzzing loudly. Because of the chaffed wire, even with the switch off, 277v was still being applied across the m-coil in the contactor causing it to buzz but not start the equipment.
These boats are set up so equipment wont trip if there is a single ground fault. Instead, the indicator lights will show problem and it is up to the engineer to diagnose and repair the problem when conditions allow. But having equipment energize itself through the ground circuits, not trip breakers, and render the ground fault indication system inoperative really bothers me. Trying to tell my company to buy a new Delta 250KVA transformer is going to be a difficult sale but what other options are there?
For a more detailed description, each boat has two 150-175kw generators wired 480v Delta. The generator chassis is bonded to the hull and we take 3 leads from each into the switchboard. In the switchboard all of the separate chassis have grounding wires that tie them into a common terminal strip which is bonded to the hull. There are 3 small control transformers wired into Wye configuration to create a neutral tied to ground. These form our ground indicating circuit for the 480v system. This is all pretty standard. There is a single 45kva 208Y/120 transformer for lighting loads. This system bonds the neutral to same common ground terminal strip one half of the boats in the class and to a separate on the other half. There is a current transducer on the neutral to ground connection for a ground fault indication.
Everything I have described about the distribution system is standard. The lighting systems in the past have typically been Delta but modern boats have been using Wye more and more frequently. So the issue I have is once the boats are tied into the Wye shore power. The shore power is a 4 conductor cable, 3 hot and a ground that connects to the switchboard. The ground from the shore power is tied into the same terminal strip as the 480v Delta grounds. The first issue I have seen is that if you do have a ground fault on shore power, this connection has rendered the 480v ground indicating lights inoperative (they all stay lit). I'm not sure why this is, yet. But the biggest issue I see is that I have had ground faults in equipment that cause the equipment to energize at 277v through the ground connection to the neutral in the shore power transformer. I first experienced this on a hawser winch with a 480/240v brake solenoid. A screw in a junction box had pierced the insulation on a wire. The winch failed to operate on generator power because the brake would not release and the motor could not overcome the brake. The boat comes in for me to troubleshoot, we plug in, and everything works normally. The screw pierced the wire in such a way that power from L1 could flow to ground through one half of the solenoid (which effectively rewired it for 240v with half the power available). So, on shore power, L1 to ground generated 277v which was enough to drive the solenoid.
More recently on an older boat, I had a similar problem with a chaffed wire in a pressure switch controlling an air compressor. Upon energizing the controller (with the switch in the off position), the controller began buzzing loudly. Because of the chaffed wire, even with the switch off, 277v was still being applied across the m-coil in the contactor causing it to buzz but not start the equipment.
These boats are set up so equipment wont trip if there is a single ground fault. Instead, the indicator lights will show problem and it is up to the engineer to diagnose and repair the problem when conditions allow. But having equipment energize itself through the ground circuits, not trip breakers, and render the ground fault indication system inoperative really bothers me. Trying to tell my company to buy a new Delta 250KVA transformer is going to be a difficult sale but what other options are there?
