ECN Forum Forums Technical Discussion Electrical Theory and Applications Neutral bonding in separately derived systems

Hello guys!

In the “theoretical” case that “maintenanceguy” presents, with a transformer in which it’s secondary neutral is not bonded to ground, we will have a non-polarized system, just like many in South-America and, I think, in Europe.

OCPD will keep tripping in case of overcurrents, and differential breakers (GFCI) will do it for ground fault currents. You must keep in mind everything is now powered Line-to-Line.

In this systems there are not “Neutral”. All lines are Hot, but voltages and currents still comply with Kirchoff laws, even with respect to the floating ground.

In fact I think that the use of the “Neutral” system is for greater safety (less hot lines), nothing more than this. Over there, in those countries (like mine), the line connected to the central point of a Wye transformer is named “Common” not “Neutral”.

Joe.-

Way back when I was a little helper, we wired a warehouse with rows of fluorescent lighting. We used a 480-277 to 208-120 transformer to feed a 3-phase MLO breaker panel.

We ended up not bonding the panel's neutral bus until last, and when we did, one of the fixtures went out. I immediately deduced that a ballast wire was pinched, and I was right.

Until this connection was made, the separately-derived neutral's voltage was actually above earth by the output voltage of the ballast, somewhere around the 600-800v. range.

Bond those neutrals; it matters.

We always bond our neutral to something, but usually not Earth. Most of our secondaries are bonded to "Structure" or "Structure ground". Utilizing the isolation properties of the transformer can help prevent thousands of amps of DC from seeking an alternate return path. I can see other times where you would be bonding to a power supply for cathodic protection.
Joe