Let’s first define what exactly means Grounding and Earthing. The fact is that the meaning of “ground” is usually associated with “earth” (soil) so for many there is no difference between these two words. However, in the “electrical world” these are two different animals that should not be confused. It’s true that both – “grounding” and “earthing” represent connection to the earth. In practice such connection will be implemented by a robust wire connected to the rod (stick, plate etc..) inserted into the ground (earth) well below the surface. What makes the difference is where the opposite end of the wire is connected.
In the Grounding scheme, the opposite side of the wire will be connected to “active” (current carrying) part of the electrical equipment. For example, in the 1-phase transformer it will be one of the wires, in Split-phase transformer it will be the center tap. Such connection will set the potential of that point to “zero” volts in reference to earth (GND level). This “reference” is then distributed downstream the electrical system by the Neutral Wire. However, since the Neutral wire makes part of the active system (carries the current), the initial “reference level” (GND) will be slightly affected (increased) by the voltage drop along the Neutral wire. Nevertheless, in well designed (not overloaded) electrical distribution system, the potential of the Neutral wire at the far-end will be not higher than few volts above the initial reference (GND) level.
In the Earthing scheme, the opposite side of the wire will be connected to metal parts of the electrical equipment (body), setting its potential at the reference (Earth) level. In difference to grounding, now the “earthing” wire is NOT connected to the current-carrying wire (they are separated by design and insulation barrier). The earthing system protects humans from electroshocks in case of fault (when “hot” wire accidently touches the metal body)
Such scheme was widely used in the past when “ground-wires” were not part of the residential circuits. Many may still remember wires connecting the body of a fridge or of other heavy loads to water pipes (note that the vast network of metal water pipes makes an excellent earthing system).
Since long, modern home-circuits include GND wires effectively replacing earthing home equipment. Rightly called Ground Wires, they make connections between metallic bodies of home appliances, light fixtures etc, but instead of being connected to Earth, they are connected to the system Neutral Wire coming from the transformer.
In all mentioned protection schemes, under normal circumstances Grounding and Earthing wires do not carry any current. They provide the path for a current ONLY in the case of short in the system till the moment (usually milli-seconds) when one of the circuit protections (breaker, fuse of GFCI) disconnects the faulty circuit from the hot wire.
The concept of Grounding and Earthing. Source: Circuit Globe
Summarizing: In modern electrical installations, the main concept of Earthing bodies of appliances was replaced by Grounding Wires. However, for the safety reasons, the earthing still holds for metal housing structures. Robust, lasting connection of container’s body to Earth, fixes its potential to “zero” volts, so in the case of fault (short-circuit to container’s wall), it will protect humans from potentially fatal electric shocks!
Note that even in the case when the local transformer (portable Power Generator or Inverter) is not grounded (its current-carrying neutral wire not connected to the ground (earth)), protection in terms of safety will still work. While breaker or fuse may not act, because the short will not close the return path through the earth to the Generator’s transformer, at least the container’s body will stay at the potential close to that determined by Earth. And just to answer the question – the high voltage caused by the short will appear between: ground (earth) – transformer’s body – and its interior wiring. It will be as dangerous, but in this case away from the container! (more on that below).
a) Container Houses connected to the Grid
Modern-day electrical service is connected to your house by a cable including 3 wires, correspondingly two live wires (hot) and one Neutral wire. The live wires are connected to the transformer in such a way that their respective voltages are exactly at the opposite phase (180 degrees). The neutral wire is connected to the “Center Tap” of the transformer. In such configuration (known as “split-phase”) the voltage between both live wires is 220Vac, while that between any live wire and the neutral one is 110Vac (North American system). The 220Vac is used for high-power loads (washing machine, dryer, range, A/C etc,) while 110Vac for lights and outlets serving smaller appliances.
While in 220Vac circuits the return current is carried back to the transformer alternatively by the live wires, the neutral wire provides return path for 110Vac circuits.
Note: Here the notion of return path is used symbolically, because in AC (Alternative Current) systems, direction of the current flow changes at the frequency of 60 Hz (50Hz) following the sinusoidal shape of the voltage). The name “Neutral” indicates the fact that the wire is at the potential close to “zero” (earth).
The Center Tap of the transformer (place where the Neutral Wire originates) is connected to the Earth (in this case to the robust metal bar/rod/pole buried in the earth. This low-resistance connection guarantees that the transformer’s center tap is at earth’s potential (considered as reference, “zero” or often “ground” (GND) level).
Typical US pole-mounted distribution transformer providing “split-phase” power for residential service (rated 120/240 V). Visible are 2-hot wires and 1 neutral wire connections and (on the right side) Earth connection. Source: Wikipedia
If wires are properly selected, the voltage drop between the transformer (source) and user’s loads is relatively small. Practically speaking, it wouldn’t be “visible” at the user’s 110Vac (hot) end. When it comes to the Neutral wire, the situation is different. Most of us will expect it to be at the potential of the GND (Earth). However due to flowing “return currents” and wire’s non-zero resistance it’s potential at user’s end will be higher (typically few volts in reference to the Earth level). However, because such low voltages are safe for humans, under normal circumstances the neutral wire does NOT represent any danger.
At the user’s side, Live and Neutral wires are distributed across the house to individual loads feeding lights and appliances. From the safety point of view most appliances and light fixtures with exposed metal parts (metal housings) must be connected to GND with the help of the third wire (usually bare copper wire in Romex-type cable, or individual green wire). Such appliances have power cables terminated with 3-prong connector. On the load side, the GND wire is connected to an exposed metal housing (fixture etc) while at the Main Power Panel, it is connected to the Neutral Bar (point where the Neutral wire coming from the Power Utility (transformer) is connected to neutral wires from all circuits in the house).
Under normal circumstances the GND wire does not carry any current! However, when for any reason (accident, malfunction, tear & wear…), the hot wire in a protected appliance touches exposed metal part (an event called “short-circuit”, because the load is “bypassed”), the current will flow through GND wire back to the Neutral wire located at the Main Power Panel and from there back to the transformer. As in such case, the current is basically limited only by the resistance of wires, it will trip the breaker or blow fuses effectively disconnecting the mentioned hot wire (and faulty appliance) from the high voltage (110Vac).
Examples of normal and faulty (short-circuit) operation of the residential 110Vac/220Vac system (unfortunately, here the Earth connection on the transformer side is not shown). Source: Larry Dimac (TheCircuitDetective.com)
Note that some (especially small appliances) are designed with a “double Insulation” (in practice it means that either the appliance does not have exposed metal parts or the probability that the interior “Hot Circuit” will touch them is close to zero. Such appliances do not need GND-protection and so their power cables are terminated with 2-prong connectors.
Summarizing this section: All modern electrical installations use GND wire(s) to disconnect high voltage (by blowing fuses) from the circuit where a short occurred. This minimizes the possibility of electroshocks and the probability of fire.
Now we are ready to discuss the problem of Earthing (or not) electrical systems at the user’s side. Unfortunately, here there are many misconceptions mainly due to practices dating to times when GND wires were not mandatory. In modern electrical systems the return path for the current is provided by the Neutral Wire. Local GND wires (home circuits) provide “emergency” return paths from the faulty appliance (load) to the Main Power Panel where the GND wire is connected to the Neutral one. The whole electrical system is balanced, because the current flowing into the “house” and leaving it back to the transformer is contained in the same cable (Hot and Neutral wires). In other words, what comes by the cable, also leaves by the same cable.
Now imagine the situation, when we connect to earth the Neutral Bar on our Main Power Panel. As discussed above, the Neutral wire is earthed at the transformer side what sets its potential to “zero” at the source. At user’s side (due to the voltage drop along the neutral (return) wire), its potential will be higher than zero. Connecting it to the Earth will open a second “parallel” path for the return current (Neutral wire and Earth path). The network will become unbalanced and, in a way, “uncontrolled” because while the resistivity of Earth is small, it is not well defined (depends on the type of soil, level of moisture etc..). As the result – the “Power Transfer System” will lose its symmetry, essential for many protective devices installed down the line. Another unwanted effect will be the presence in the soil of so-called wandering currents (currents will follow the path of “least-resistance” rather than straight line to the transformer).
Bottom line: At the user’s side (house) the Neutral and GND wires should be NOT connected to the Earth! Earthing current-carrying installation should be done Only at one point (and in this case it is done in the transformer).
Does it mean that we should not connect our house to the Earth? For traditional housing structure, the answer is clear – NOT (or rather, there is no need for that, after all, what would you connect to the earth? Brick walls?). However, for metal structures (and that’s where we come to container houses), the answer is – YES (see “earthing” section below).
As Murphy’s laws predict, if something can happen, it certainly will. If for whatever reason a hot wire touches the container’s metal structure (wall, studs etc), it will expose dwellers to electroshocks. Especially, touching the wall from the outside (standing on the wet soil) will open an electrical return path to the transformer through the human body with disastrous consequences.
See what happens when the hot wire accidently touches container’s body that is not connected to the earth (here represented by the “Electrical Equipment”). Source: Circuit Globe
And similar situation, but when the container’s body is earthed (connected to the earth). Source: Circuit Globe
As it can be seen however, earthing container’s body does not create any permanent return path for the current. It’s because container’s body is isolated from current-carrying circuit. The return path may be created only temporarily by a short-circuit. But then the breaker, fuse or GFCI (RCD) will automatically disconnect the hot part of the circuit from the faulty equipment.
b) Off-grid Container Houses
Many of the above concerns and restrictions do not exist in low-voltage systems (voltages lower than 50Vdc). Obviously, all low-voltage circuits need similar protections (fuses, breakers etc… ) to quickly eliminate short circuits (if any) and prevent fires. The main difference is that eventual exposure to the voltage lower than 50Vdc is not dangerous for human life.
The major problems of transferring power at low voltage are related to significant voltage drop along wires, associated loss of power and overheating (as discussed earlier).
Due to the DC (direct current) nature of most off-grid Power Systems (solar as well as small wind and hydro generators), things are more straightforward. Hot wire feeds the system with energy, while the Neutral wire unambiguously provides the return path (hence the terminals are typically identified as “+” and “-“ ( instead of “Hot” and “Neutral” like in AC circuits). The bottom line is – low-voltage circuits do not need grounding wires!
- In practice, many off-grid containers houses will also use AC systems. They can be based on:a) Portable diesel generators supplying energy in the time of peak demand or unfavorable weather conditionsb) Inverters – in this case the AC power is available from DC system (including energy storing batteries) and on-demand converted to one of AC standards (110Vac/60Hz in North America and 220Vac/50Hz in the rest of the world.)
In both cases, these will be mostly Single-Phase AC systems (2 or 3 phase units may find use in off-grid commercial and industrial applications). That brings us back to almost everything we already wrote about connections to the grid and mandatory safety requirements (GND wires and Earthing). All that applies!
“Almost”, means that there may be one exception – the earthing system. If the Generator (Inverter) has an Earth Connection, it’s the same situation as connection to the municipal grid. However, if the Diesel Generator (or Inverter) is not earthed at its “neutral-wire” terminal (probably illegal, but unfortunately possible), earthing your container’s structure will not create the return path back to the source. As a result, the breaker will not trip, fuse(s) will stay intact, GFCI systems may not work and the high AC voltage won’t be disconnected from the faulty equipment with all related consequences (see the earlier discussion regarding differences between Grounding and Earthing).
That’s why, for security reasons (protections from electroshocks), container’s metal structure must be connected to GND wire(s) (usually multipoint connections). Note, that without grounding/earthing at the source, the potential of the neutral wire will be somehow “floating” compared to the reference level determined by the earth.
In such case, despite the fact that “earthing” does not help the container to create a current return path to the source (Generator or Inverter in this case) it is still recommended. By connecting GND-ed container’s structure to the Earth we create a Single Reference Point in our home AC system defined by the location of the earthing rod. As the result, the container (and the AC network) is not “floating” at some random potential (voltage) but is set to “zero” (earth reference).
Note that isolated structures (floating) can be randomly charged by electrostatic energy from air (wind). Touching them, when standing on the soil (earth), our body will serve as the discharge path with certainly “unpleasant” (if not dangerous) consequences.
Well, frankly, there is much more to say about wiring and safety in combined AC and DC networks that most likely will be used in off-grid locations powered by renewable DC power sources. It will be subject of another article.
Earthing the container
Many may think that container is “naturally” earthed just because it is placed directly on the soil, or on concrete blocks. The truth is that such “earthing” does NOT meet relevant specifications (and functionality when needed) and it won’t be accepted by local authorities. For that you should use a large metal bar/rod deeply inserted (pounded) into ground to guarantee a reliable, lasting, low resistance connection.
Earthing container’s metal structure guarantees that it will always be at “zero” (Earth) potential. In the case of “short circuit” to container’s metal structure, the current will flow through the earth to the transformer, tripping breaker or fuse(s) on the hot side of the wire and/or any “upstream” GFCI (RCD) devices.
a) Grounding the container
Grounding container’s metal structure (understood as connecting it to interior GND wire(s) will also protect from electroshocks. In the case of a “short-circuit” to the container’s metal structure, corresponding fuse will be blown. However, such solution has some disadvantages compared to earthing:
– Theoretically, the container’s structure (walls…) will not be anymore at “zero” potential of Earth, but “floating” at some low “non-zero” voltage following the potential of the Neutral wire. It won’t be dangerous, but also not “overwhelmingly” recommended.
– As mentioned above, due to large metal structure and exposure to “surroundings”, container houses will usually have some “uncontrolled” (random and un-characterized) connection to earth. Whatever will be its resistance – few Ohms or hundreds of Ohms, it will provide an extra return path to the transformer we wanted to avoid.
– Note that the transformer is usually far from your house, with many other houses (customers) in between. The value of non-zero potential at the user-end of the Neutral wire depends on return currents from all these customers. High currents will make it higher!
Summarizing Grounding and Earthing:
– Containers connected to Grid
It is assumed that the power source (Transformer) is grounded and earthed. In such case, container’s structure should be not connected to GND (wires). However, for security it must be Earthed.
– Containers connected to local AC Power sources (Portable Generator or Inverter)
If AC source is grounded/earthed (similar way as Grid), then the same rules apply when it comes to container’s structure: No grounding (GND wires), Connection to Earth (Earthing)
However, when local AC sources are not grounded/earthed, then one common GND/Earth point in the AC system must be established by connecting container’s structure to electrical GND wire and then to the Earth (Reference).
– Containers with only low-power DC systems
One “side” of the low voltage system should be connected to GND (in this case container structure). Typically, it will be the Negative Terminal “-“ (minus) of the battery system.
Also, the container must be earthed (connected to the Earth).
It’s important to note that lightning rod (if used) must have its own earthing system. It’s because in the case of lightning strike (even not direct one), discharged energy is so high, that regardless how low is resistance to earth of the Earthing Bar, the voltage across it will also go very high. If the Lightning Earthing System is connected to your electrical system, it will also blow your appliances and possibly the whole electrical network. But that’s the subject for another article.
The above article cannot and does not intend to replace the National Electrical Code or relevant local building codes. It is meant for informational purpose ONLY! Its goal is to reveal to potential DIY “enthusiasts” and home-grown electricians the scale and seriousness of problems related to electrical installations in container-based houses. It should give ideas as to what kind of problems you will face when designing and making your electrical installation, what you should know when hiring a certified electrician for this job etc… However, the “Bible” still is (and last words belong to) the NEC and local building codes. It’s your responsibility to follow administrative rules!
All parties associated with the above text are NOT Responsible for the use and results of this information by any party. Any hazard created when building the electrical installation based on information contained in this article is the SOLE Responsibility of the USER!