I had a home inspection performed on an old house recently. The inspector noticed that some of the outlets had an unconnected third prong. He told me that the absence of a grounding conductor on the outlet wouldn't cause any harm to the home's electrical system itself, but that modern appliances that are designed to work with a 3-prong outlet could be damaged if the grounding conductor is unconnected. He used an example of a television set and mentioned that the t.v. could be fried by an unexpected voltage transient from the grid, and then mentioned something about currents travelling in the wrong direction.

I didn't feel like starting an argument so I didn't press him on this issue, but what he was saying sounded completely bogus to me. I thought the grounding conductor was meant to protect the device user from harm, not the device itself? However, he seemed like a pretty competent guy, so I'm wondering if there's actually any truth to his claims?

  • 1
    \$\begingroup\$ "mentioned something about currents travelling in the wrong direction", well alternating current "travels" from the live conductor to neutral and vice versa 50 times each second (60 times in USA), there is no "wrong" direction in that sense. Maybe he meant "the wrong path", i.e. through the user rather than in the ground wire, in case of a failure in the appliance while the user could be touching its outer metal parts. \$\endgroup\$ – Lorenzo Donati supports Monica Jun 14 '18 at 14:13
  • \$\begingroup\$ Yes, I think he meant 'wrong path'. But, he was not speaking of current through the user instead of the ground wire in a fault situation. Rather, he was trying to explain why plugging a device with a 3-prong cord into an ungrounded outlet could potentially result in damage to the device itself, not the user. \$\endgroup\$ – pr871 Jun 14 '18 at 14:21

There are 2 scenarios where 3 pronged earth-bonded plugs are used.

  • when SMPS are a classic line filter is used to suppress outgoing noise. The noise currents are shunted to earth ground using Y caps on each line to <0.5mA of line frequency max. For safety reasons in case of ground fault and any user touching the metal ungrounded frame and another metal object like a metal plumbing or cable TV does not feel more than a tickle of current.

The line filter is however bi-directional filter and also used to attenuate ~ 3KV power line transients from switching grids or lightning to a lower stress level for components. However the TV was a bad example as most in North America now are all 2 pronged plugs. A PC would be a better example. Not having an earth bond to now relies on the Y cap to attenuation the transient applied to the Common Mode choke. This may or may not be effective in protecting the equipment but now connects the metal case to the line thru the Y caps to a make a unsafe lightning voltage transient on the case.

  • The other scenario is old equipment which do not have double insulated cases like metal hand-held saws and drills and again if the insulation breaks down creates a risk to the user during power line transients.

Earth ground bonding is more a human safety protection 1st.
- 2nd an equipment protector by attenuation for most but not all (eg Florida) lightning storms
- 3rd an EMC compatability noise filter for performance , noise on AM radios etc nearby or noise glitch resetting / locking up / crashing PC

The better solution is to ensure earth bond wires are supplied to all 3 prong outlets and use additional line filters with MOV’s for protecting valued equipment in Florida. ( the lightning capital of North America )


No, the ground conductor is intended to protect the user should a single fault in the insulation of the mains occur. Appliances which use the ground wire will have it connected to the metallic case (if it has one) or any metal parts the user can come into contact with, and should a fault occur and the live wire come into contact with any of those parts, a current large enough to trip the circuit breaker (or RCD/GFCI) will flow. Some appliances don’t have a ground wire on their plug, this means the mains voltage is double insulated, meaning a single fault will not cause the user to come in contact with mains voltage.

This means if the ground wire is not connected when an appliance that relies on having a ground connection for safety is plugged in, only a single electrical fault is necessary for the user to come into contact with high voltage.

  • \$\begingroup\$ Most circuits in a home don't have GFCI, only those that are required need GFCI ecmweb.com/code-basics/… \$\endgroup\$ – Voltage Spike Jun 14 '18 at 15:28
  • \$\begingroup\$ Yes, I included them for completeness sake, and because some houses have a whole house gfi protector \$\endgroup\$ – C_Elegans Jun 14 '18 at 16:37
  • \$\begingroup\$ Most houses do not, and products are designed for worst case \$\endgroup\$ – Voltage Spike Jun 14 '18 at 17:43
  • \$\begingroup\$ I believe I also mentioned that the fault current would trip a regular circuit breaker, again I only mentioned the GFI for completeness sake as it would trip faster than a regular breaker \$\endgroup\$ – C_Elegans Jun 15 '18 at 0:07
  • \$\begingroup\$ Yeah, sorry about that confusion, I just saw the parenthesis \$\endgroup\$ – Voltage Spike Jun 15 '18 at 2:45

The ground is there to protect you in the case of a fault most circuits don't have a GFCI on them, if there is a fault the circuit can still protect you. First of all Electronic Test Labs (ETL) like UL require you to fuse your product if you have AC mains running into it (I think an exception to this may be if it's double insulated, but I'll have to check on that). The second thing that has to happen is you need a chassis connected to ground in your product so if there is a fault AC mains will ground out and trip the breaker (and the breaker doesn't need to be GFCI).

The second means of protection is a fuse that is also required in products by an ETL, this is set so it will blow in the event of a fault.

Another benefit of grounding the chassis, during a fault if the case of the product is touched, you will not get electrocuted. If you remove the ground in the system and there is a fault, you will get electrocuted.

Ground's aren't just a good idea, they are required in both homes and products to keep houses from burning down and people from getting electrocuted, the national codes and product requirements are there for a reason and work together for protection.

Normal Operation

enter image description here

Fault Operation

enter image description here
Source (both pics): http://www.diyaudio.com/forums/diyaudio-com-articles/163575-audio-component-grounding-interconnection.html

In the sense that the inspector was talking about equipment malfunction in the event the ground is disconnected, he was right. Some surge protectors use ground to shunt voltage spikes away from devices with metal oxide varistors. If the ground is disconnected the varistors will not be able to shunt high voltage spikes away from a device properly.

enter image description here
Source: https://zerosurge.com/surge-suppression/

  • \$\begingroup\$ So was the inspector right or wrong when he said that a device (such as a t.v.) with a 3-prong plug can potentially be damaged if plugged into an outlet with an unconnected grounding wire? He wasn't talking about danger to the user, he specifically mentioned that the device itself could be damaged in the event of a grid fault. Does the grounding conductor provide any sort of protection for the device itself? \$\endgroup\$ – pr871 Jun 14 '18 at 16:04
  • \$\begingroup\$ There won't be a fault current if the ground is disconnected, the case of the device will be lit up to mains voltage and if there is another pathway, the mains voltage will choose that pathway. The devices will most likely function like it always does, but it will create a safety hazard. I edited my question with the grid fault condition. Another problem is with no ground the fuse will not work properly in the event of a short. \$\endgroup\$ – Voltage Spike Jun 14 '18 at 17:55

Lets put it into simple terms: by the time 3rd wire comes into play your device is already damaged.

So, whatever your inspector told you was just a load of baloney designed to scare you into (absolutely reasonable) compliance with the code. Many people do not believe in harm to themselves until it happens, but everybody will think twice if threatened with financial loss.


It seems my simple answer was not simple enough. So, let me reiterate:

  1. Ground wire was introduced for additional safety of the users, not the devices.
  2. Neither old nor new appliances could be damaged by plugging into outlets with disconnected ground wire. They can become safety hazards. They can emanate more EMI. They might become more susceptible to lighting or ESD. But they will not be damaged by plugging into said outlets.
  3. Inspector was wrong. As in: the butler did it :)
  • \$\begingroup\$ No, it is not already damaged, I've seen plenty of devices that were protected from a fault with a fuse. You fix the wiring and go on your merry way. This is especially important in production environments \$\endgroup\$ – Voltage Spike Jun 14 '18 at 18:03
  • \$\begingroup\$ @laptop2d I am trying to give a simple answer, which seems to be what OP is looking for. For many people blown non-resetable fuse inside a device is no better than broken device. What "production environments"?! We are talking about the house, for God's sake! \$\endgroup\$ – Maple Jun 14 '18 at 18:15
  • \$\begingroup\$ Also your "fix the wiring" somehow assumes that the fault is caused by the wiring, which is not always the case. Cutlery dropped into a toaster is one example. Water damage is another, quite common, cause. In both cases your device might be only temporary malfunctioning but the grounding will protect yourself, not the device, which was exactly my point. \$\endgroup\$ – Maple Jun 14 '18 at 18:21
  • \$\begingroup\$ Electrical devices are used in commercial, industrial and residential environments. In a production environment, if someone causes a fault current, it blows a fuse (or breaker) and protects the device (from a fire, and protects the user from shock). Let us change fix the wiring to "fix the fault" if your keen on redefining \$\endgroup\$ – Voltage Spike Jun 14 '18 at 18:30
  • \$\begingroup\$ Please, give me an example how someone can cause fault current in a residential environment that will threaten the device rather than user. \$\endgroup\$ – Maple Jun 14 '18 at 18:39

If an appliance has a metal case or other exposed components that might accidentally become connected to mains voltage (either directly or via leakage path), a grounding conductor can provide a safe path to ground, thus minimizing the current that would flow to ground via other means (e.g. through the body of a user). If the leakage path is slight, the grounding lead will prevent the body of the device from reaching a dangerous voltage. If the leakage path is strong, the grounding lead will conduct enough current to trip a primary over-current device (typically a breaker).

In cases where it would be impractical to add a grounding lead to an outlet, building codes in many places offer an alternative, called a GFCI or (Ground Fault Circuit Interrupter) in the US, or an RCD (Residual Current Detector) elsewhere. This device will detect whether any current which is flowing into the device from the mains is flowing out of the device via any means other than the associated neutral return wire. If so, the GFCI/RCD will trip and disconnect the device from the mains. If a device develops a fault which would cause the case to become electrically live, a GFCI might not trip immediately if there is no outside path from the case to ground, but if a user touches the case in such a way as to create a path, a GFCI will disconnect current in less than 0.02 seconds, minimizing the likelihood of injury.

Note that at least in the U.S., outlets that use a GFCI instead of a grounding wire must be marked "No Equipment Ground", and such usage is only allowed in cases where proper grounding would be impractical. New installations must have grounding wires, but upgrading adding a GFCI to an older installation which has no protection would make it much safer than leaving it as it is.

  • \$\begingroup\$ As much as I appreciate the good information, the question was more about whether or not the claims of the home inspector were true. He stated that a modern electrical device (such as a new t.v.) that was meant to be plugged into a grounded outlet could potentially be damaged if the outlet's ground wire was unconnected. He said that without that grounding connection, a power surge could possibly damage the device. I always thought that the grounding wire was meant solely to protect the user, not the device itself, so I was wondering if his claim was true. \$\endgroup\$ – pr871 Jun 15 '18 at 0:12
  • \$\begingroup\$ Ah. Well, another slight issue is that some surge protectors clamp line to ground and neutral to ground, but not line to neutral. A missing ground would allow twice the normal clamp voltage to reach the device. A surge would trigger a GFCI and cause it to attempt to disconnect, though whether a GFCI would be able to interrupt the surge current quickly may be another matter. \$\endgroup\$ – supercat Jun 15 '18 at 1:25

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