Neutral Grounding on Secondary of Transformer


Thread Starter

Minnesota Bob

Maybe I have just confused myself all these years and it is really simple but I don't see it. We have always grounded the neutral on the secondary of any transformer we use in our panels. I have two questions for someone related to it.

First: I can find no reference to what is required for the grounding of the neutral on secondaries in the NEC or NFPA79. I am never sure what to ground or even if it should be grounded. Can anyone directed me to the section of the code requireing this or not?

Second: Why is it grounded? I know without it you get floating voltages but I try to analyse what it is there for and get confused. It seems that tieing a current carrying conductor to ground would short it out. Is there any mathematical models I can look at to explain what is going on?

curt wuollet

I can't address the former Because I don't have the NFPA docs here
but as for the latter:

Consider that the secondary is insulated from the primary and thus
the electrical grid. So grounding any _one_ point on the secondary
isn't shorting anything, that is, causing current to flow, neglecting

If you ground two points, you have a short. But by grounding
the neutral you are referencing the "hot" wires to ground so that
they will have nominally equal and definite voltages in relation to
ground. There is an exception with the high/low leg 3 phase hookup,
but even there the purpose is to have a definite relationship between
the current carrying conductors and ground. This is for safety reasons
as well as lightning protection and a whole host of issues with a
floating secondary.

Tying the neutral to ground gives the system a reference point. Your system will then be protected against earth faults which would allow circuit fuses to blow to protect each sub-circuit should an earth fault develop. Without the neutral earth connection an earth fault on a given sub circuit would not result in a fuse blowing / breaker tripping thereby potentially causing associated metalwork to reach dangerously high potential and possible danger of electric shock should a person happen to touch the live metalwork and live parts at the same time.

bob peterson

On the secondary of a low voltage (like a 480V delta) transformer?

It is rare that a fault to earth would trip a CB or other OCPD on an ungrounded electrical system (unless it has ground fault protection built in).

Even on a grounded electrical system the fault current through earth might not be high enough to trip the breaker.

Think about the numbers for a second. What is the typical resistance through earth? 5 ohms? 25? In most cases there is no code requirement to have any particular value. It could be 10000 Ohms and be code compliant.

Say you have a 480/277 system, solidly grounded. You have a fault to earth from one of the phases. Suppose it is a 100 A breaker. What is the minimum earth resistance that would be required to reliably trip the CB? You only have 277V to neutral in such a case, and you will need about 5X the rated current to reliably trip the breaker fast enough to do any good. What's that work out to - about 1/2 an Ohm? Does anyone have an earth resistance even close to that for this type of electrical system?

The non-conductor but metallic parts of the electrical system are a completely different story than the earth itself.