3 Red Light Bulbs in an Industrial Facilty If one went bright or went out it meant big trouble in the old days

Its been a while since I have seen  this 3 Red Light Bulb set up till I walked into an old building which served as a Car Dealership to do some work  and there they where.
Since I am an older  electrician the customer asked me if I knew what those 3 bulbs represented  in the repair shop area. Yes I  told him they where used to warn if there was a problem with the 230 volt 3  phase delta  power coming into the building before modern 3 phase monitoring relays .
I have also seen where sometimes 3 yellow bulbs have also been used.
The 3 bulbs  warned of  problems with the 3 phase  if a bulb went out  there was a usually a phase failure and you needed to shut down the machines or destroy them..

If one of the bulbs went brighter than the others it meant one of the phases was grounded out some where in the shop.  This could be potentially dangerous and fatal . But many shops would still run and then when they had time  at end of a shift go and look for it machinery at that time was not grounded with ground wires.  This was common practice till late 60's  when electrical engineers determined it was safer and better to run ground wires to all equipment and trip out a motors overload relay  if theirs a grounded condition in the motor   VS.  not grounding machinery . which could allow an entire line of motors to be destroyed under the right circumstances. Besides damage to equipment the machines operator if he was touching a machine and  would become grounded by not standing on rubber mat under his feet or touched something else metal could receive a fatal electrical shock under the right conditions.
 With modern electrical codes the practice was all but abandoned by late 60's  but you can still find some facility's using the bulbs to warn of a phase failure. At the now gone  BSA  camp Semiconon in Butler county Pa. they installed the bulbs near the camp office to warn when 3 phase was out to the camps water pump which was frequently plagued by single phasing conditions.