Inadequate grounding of sub panel

Question

Three wire system. Twenty years ago i installed 50 amp Sub panel at the garage connected to house panel. It supplies 3 circuits at 15 amps, 1 at 20, a double 20. There is no ground rod at the garage. Yesterday the circuits from the sub panel went dead. I measure 220 volts across the main lugs, but zero from each lug to the ground cable/bus. T garage. The cable from the house panel to the house penetration is aluminum and it then connects to copper. I am thinking of driving a ground rod at the garage to connect to the ground bus in the sub panel. Am also thinking that corrosion at the Al to Cu connection has essentially opened that connection.

Install the ground rod and connect to the ground bus or to just the supply cable ground? Install it but disconnect the supply cable ground from the ground bus. Open and clean the Al to Cu connection.

Answer 1

Summary: a ground rod will not substitute for the neutral/ground connection back to the main.

To be clear, your third wire is actually providing two functions: neutral and safety ground. The power disturbance is due to the failed neutral. 120 v loads on only one leg do not have a return path and so will not work. If you have loads on both legs, they will conduct through each other but, depending on the ratio of the loads, the voltages will vary between nearly 0v and nearly 240 volts and may destroy equipment or cause a fire.

The safety ground has multiple functions but significantly is to prevent shock with metal-cased, three-prong equipment. If a hot wire were to come lose and contact the metal case, excessive current would flow to ground and trip the breaker on the hot. If a GFCI is in the circuit, the trip will come with much less current. This prevents fatal voltages from appearing on the case.

In this case, because the neutral and safety ground are combined and have become disconnected, you are actually in worse shape. Voltage will conduct through switched on loads and actually energize grounded objects (metal-cased 3-prong equipment, outlet and switch screws, etc.) providing a great shock hazard.

This is why the electrical code has been changed to require a 4-wire feed to sub panels.

A ground rod would mitigate this to some degree but dry dirt is a poor conductor. It is a poor safety ground and a lousy neutral. You absolutly need a working neutral/ground back to the main panel.

At a minimum, I would fix the open conductor and add a ground rod at the garage. You should also consider upgrading your feed to 4 wires to separate the neutral and ground, as current code requires.

Answer 2

Don't worry about the aluminum wire, given the ampacity it is surely #4, and aluminum wire of that size is definitely alright even if it's the old AA-1350 alloy. It's probably the new AA-8000 given the 1998 vintage. Clean it up, it should be fine.

The 3-wire configuration was legal up until 2008. Effectively it is legalized "bootlegging ground". As with 3-wire dryer and range circuits, there's a risk, and I gather you've "shaken hands" with it. Ground rods won't fix that. You need to carefully maintain the neutral wire to prevent this problem.

If you want the redundant safety of a ground, the only workable option is to retrofit a ground wire back to the house.

A grounding rod cannot perform that function at all. Dirt does not conduct electricity to any useful degree.

It conducts just enough to cause some milliamps of flow if you touch a hot wire while standing on the ground, enough milliamps to kill you. We reduce that risk by assuring the electrical supply is near earth potential so less current will flow, that's what the neutral-ground bond in your main panel does.

The grounding rod is useful for sinking natural electricity, notably ESD.

Retrofitting ground

This can be done as "just a ground wire" and need not follow the same path as the cable. You connect your subpanel's chassis or grounding bar to your main house's main panel's ground bus or grounding electrode.

Then, you will need to electrically separate all neutrals and grounds in the subpanel. If you are used to just spamming all neutrals and grounds onto the same bus, this will come as a surprise. Some panels come with two bars linked together with a neutral-ground bond which can be separated to make 2 bars. Other panels require purchasing an accessory ground bus and removing the ground bond or screw from the neutral bar.

Any bootlegging of ground anywhere in the garage must be removed. Neutral cannot connect to ground anywhere in the garage (except inside a faulty appliance, but with any luck that will trip the breaker.)

This, and only this, will protect you from shock if the neutral fails again.