120-240 delta service question

| I work for a commercial electrical contractor and we have several | property management companies as regular customers. | | We are required to replace main circuit breakers or test blocks in | services on a fairly regular basis. Most of the time, it appears that | the main circuit breaker or test block was burned up due to alum. | wiring installed from the test blocks to main c/b then from load side | of main c/b to panel. | | We of course replace all wiring with copper and replace test bllock or | main c/b as required. | | The question I have is it appears that most of the problems appear to | occur on 120-120 delta services. The high leg is usually the burned | (or most burned in some cases) leg. | | On a high leg system, there is usaully a smaller load on the high leg | (Atleast in the case of these smaller tenant spaces where there is | limited 3-phase or 1-phase 240V equipment). This would lead me to | reason that one of the other legs would burn up due to load imbalance | more frequently that the high leg.

What are the ratings? Are the 3 legs all rated for the same current? You'd think the high leg loads would be less, but can you be sure? If a 240 volt motor is connected, it would draw more current or stall at times.

| Is it just coincidence, or is there a reason why the high leg would | cause such a problem?

A 240 volt single phase motor could be pulling more current than expected on 208 volts, if it is connected between high leg and ground (instead of between the two low legs). It could even stall. But I'd think this would mostly just trip the branch breaker to it.

If there is a phase loss, or total power loss, the backfeed from motors spinning down could be problem, too, if they don't disconnect.

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The only problem with AL is the person installing it. I have installed AL for 35 years with never a failure. All the US utilities use AL they can not be all wrong.

I will bet your failures are workmanship, not material.

Are the breakers really rated for delta, not 3 phase but delta 3 phase?

Test blocks that I have seen and installed are not full current load devices. They allow access to the CT's and PT's on the secondary side. All the ones I have seen are wired with #10 solid. I sure would like a picture of your test block (s) so I could further my education. Looking forward to the pictures.