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Sounds like the 70 amp breaker was in fact tripped but the handle just didn't indicate it. I see that occasionally. As far as why it tripped that is unknown. Perhaps the inrush current was too much for it and it tripped, maybe the breaker is getting old and needs to be replaced, there could be something going wrong with the heater unit or the heater supply circuit. For now, keep an eye on it. If it does it again then you know you have a problem.
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