Before putting our #471 to bed for the winter, we ran a self load test. As fate would have it, one of the grid resistors burned out before we got to a full load - 960 amps in run 5 was as high as we got, it burned out when moving to run 6. I’m not an expert on these grids. On close inspection, the grid that popped is significantly different than the other three. The ribbon appears to be thinner in gauge and is much longer if you stretched it out. This may be the one with a fan motor tap, don’t know yet. We can’t work on it until next spring so I’m looking to get a little educated in the meantime. My question is, is this difference in grid resistors normal?

Background info on the unit: Originally a non-dynamic, MEC GP7 of 1950 vintage. Rebuilt by Guilford/MEC in 1985 with the addition of dynamic brakes (with self load test panel) and a “super crew cab”. Completely rebuilt (remanufactured may be a better term) by Amtrak for CDOT use in 1994. It’s essentially a GP38 in sheep’s clothing.

"Normal" is difficult to define in this case. Not all the companies selling dynamic brake grids were OEM suppliers to EMD or GE. Some are strictly aftermarket sellers with their own designs. Products from multiple manufacturers may or may not get along well in the locomotive electrical system.

Best practice is generally that all dynamic brake grids in a particular locomotive should be same supplier and same construction. The concern is that mixing grids with different air flow characteristics will result in some grids running hot.

PC

Thanks. That makes a lot of sense. The one that popped would have more restrictive airflow due to closer ribbon spacing. It must have been a quick change out with whatever was on hand by a previous owner. This was the first time we’ve load tested since we’ve owned it.