CT Pow

Control transformers have been around for as long as I have been designing panels. The majority of what we see in the field and our designs take single phase 460VAC to 120VAC. Folks sometimes forget to ground/bond the secondary to create the neutral. Or we get calls about the primary or secondary fuses blowing. Generally, the fuses or transformer are undersized.

Members of the AEG team were onsite at a customer’s facility when we heard that really scary “electric” movie effect sound of something, or someone, being electrocuted. No one was hurt, but the transformer and panel were completely toast.

The postmortem revealed that the junior electrician hooked the L1 and L2 phases to two terminals that were jumped. The result was a classic short circuit. And since this was a temporary hook-up, there were no fuses and the 100A service did not trip. Someone had to physically go and turn off the breaker.


Where to start? Final checks on wiring by competent people. The words “temporary” should coincide with maximum safety scrutiny.

Be aware that some transformer companies make the primary secondary connects internally and it matters where you land the cables. Other companies allow for jumpers that slide into the terminations. Always make sure the schematic that is printed on the nameplate is reviewed prior to power-up.

Last words

In the United States, on average, 400 people die from electrocution and 4,400 are injured each year due to electrical hazards.  Of these electrocution deaths, 180 are related to consumer products. Annual workplace electrical accidents account for another 325 deaths and 4,000 injuries, according to data published by the National Safety Council.  Electricity also causes over 140,000 fires each year, resulting in an additional 400 deaths, 4,000 injuries and $1.6 billion in property damage. Looking at an even greater perspective, economic losses due to electrical hazards are estimated to exceed $4 billion annually.