Surge protectors are banned on cruise ships, and so are confiscated during boarding. Extension cords and cube taps etc. are discouraged, but not banned.

I'm asking specifically about surge protector incompatibility, and would like an answer that includes the technical details of the ship's cabin power supply.

I've asked in various places (not StackExchange) and have never been able to get a technical answer as to how a surge protector can cause damage.

Most responses are along the lines of "they cause fires", "fire is really bad on a ship", and "they ban all extension cords", so please don't give a similar answer.

To discourage "they ban all devices" responses, please look at this ad from Amazon and note the "Non Surge Protection & Ship Approved":

enter image description here

(Note that I wrote this last night before the answer from @StainlessSteelRat)

Learning that ships use an isolated-neutral system was a great help.

Here is a summary (many details omitted) of where I am so far. In particular, I run into what seems like dead ends (in bold) where the bad effects aren't nearly as bad as they at first sound.

Wiring Systems:

  • Normal house wiring (North American standards) is asymmetric, having one line at 120V, and the other shorted to ground at the main breaker panel. The neutral side (larger slot in outlet) is, in theory, safe to insert a fork without shock. The live side is, in practice, dangerous to insert a fork.

  • Ship wiring is symmetric, having two lines, each at 60V and 180 degrees out of phase. Both sides in theory pass the fork test (similar to birds safely sitting on a distribution wire).

Surge Protection:

  • With normal house wiring, when a surge protector notices a voltage spike between ground and the live wire, it diverts the excess current.

  • a surge protector has a continuous but small current (for the detection), and occasionally a brief but large current (when clamping).

Surge Protectors on Isolated-neutral systems:

  • The connection between one of the live wires and ground causes the system to behave like a grounded system. The current is normally small, but with many cabins on the same circuit, it might become significant. This makes the system more dangerous to users, but no more dangerous than home wiring.

  • The grounding occurs on the side of the circuit that connects to the live socket. Devices that rely on the difference between the two sides will be compromised. For instance, the on/off switch is normally on the live side, but would now be on the neutral side, leaving voltage on the device (e.g. even when switched off, the threaded part of a light-bulb socket would be live). This makes the system more dangerous to users, but no more dangerous than home wiring that has live and neutral reversed.

    • There will be multiple connections to ground in the circuit, which can create ground-loop currents. This can be annoying (e.g. hum in sound systems), but doesn't create any danger.


  • If the two wires aren't different colours, or if the installer knows that it doesn't make any difference, one line could end up connected to the neutral outlet in one cabin and to the live outlet in another. That could increase the continuous current leakage through ground. In the case of an actual spike, both sides would be briefly clamped to ground creating a large current and possibly tripping the circuit breaker. This would be annoying, but not dangerous.

  • I'm sure that the ship's electrical system has devices to monitor and regulate the power supply. The current leak to ground caused by surge protectors might cause this mechanism to get false readings. This could be dangerous to users.

  • There is an effect on metals known as stray current corrosion. Even without any added electricity, random currents occur in metals, and, particularly in the presence of salt, will cause the metal to corrode at points that act as anodes. Many marine vessels have magnesium-anode devices attached to their hulls to mitigate this effect. I imagine this is the reason that ships use isolated-neutral wiring, to avoid introducing any electrical currents into their hulls. This isn't immediately dangerous, but would be immensely expensive in the long run.

  • 3
    $\begingroup$ Ships use an insulated/isolated neutral electrical distribution system. Surge suppressors are trying to dump to ground . "Floating" electrical distribution can tolerate a single accidental earthing and no current will flow and no protections will blow. It takes two earthings to create the possibility for an over current fault. Each surge protector represents one such earthing path depending on the design of the unit. Surge protectors engage at a variety of voltages from 330 to 600 or so. Thus they would be fighting each other when the neutral is not massively bolted to ground. $\endgroup$
    – Phil Sweet
    Mar 30, 2019 at 22:16
  • $\begingroup$ Some units have a high leakage current to ground. Particularly old ones. This interferes with the calibrated resistance device connecting the generator "neutral" to the ship grounding bus. The idea is to let the system float enough to handle a ground fault, but not float too much. $\endgroup$
    – Phil Sweet
    Mar 30, 2019 at 23:21
  • $\begingroup$ @PhilSweet You should put that as an answer - and for info trucks with IER (insulated Earth Return) can have similar issues... $\endgroup$
    – Solar Mike
    Mar 31, 2019 at 2:24
  • $\begingroup$ My comment was somewhat speculative. I have some knowledge of small boat electrical systems and their quirks, but not cruise ships. And I'm not an EE. I'll wait for a EE to explain the risks and concerns of the cruise ship people. $\endgroup$
    – Phil Sweet
    Mar 31, 2019 at 12:26

3 Answers 3


From USCG Surge Protective Devices Onboard Vessels

A marine casualty investigation of two separate stateroom fires onboard a U.S. Flag Container ship revealed that the sources of the fires were attributed to the use of SPDs plugged into a lighting circuit. It was discovered that a ground had developed on another circuit that was connected to the same distribution panel providing power to the staterooms. This ground created an imbalance of voltage between the two power conductors supplying the SPDs which caused excessive currents, overheating, and subsequently, a fire. In this instance, even if the SPDs automatically tripped as designed, only one power conductor would have been secured while the other would continue to provide power, possibly shorting to the device’s ground wire and the structure of the vessel.

This safety alert only applies to vessels with alternating current power systems and may be most likely applicable to larger industrial and commercial vessels.

There are two main methods to supply power on ships. Insulated neutral and high impedance grounded neutral.

The insulated neutral system is used for low voltage ships (<1000V, typically 440V @ < 5MW). An insulated neutral is used to maintain the integrity of the power supply system.

On land, a grounded neutral is used. Any ground fault (hard or instantaneous) will cause breakers to trip or fuses to open. At worst, you are in the dark.

At sea, loss of power coming through narrows or channel may cause a dangerous situation. So an insulated neutral is used. A single ground fault (hard or instantaneous) will energize the hull, but much like birds sitting on uninsulated wires, the crew is not at an immediate risk, but the ground fault must be cleared because:

  • if a crew member makes contact with a second power line, they may be shocked.
  • and a second ground fault will interrupt the flow of power, possibly blacking out the ship.
  • and cathodic corrosion protection which protects the hull from rusting may be impacted.
  • and specifically may cause a fire because current greater than wires ampacity flows, I2R losses create heat, melt/burn insulation and causing a fire.

Circuit protection is designed to protect the insulation of wires to prevent fires.

enter image description here

So ground faults are dangerous and must be cleared as soon as possible. Circuits must have circuit protection (fuses or breakers) on all power lines because ground faults can develop on different power circuits.

Ground faults occur in ships in all spaces. In panels, in wiring, in devices, dirt collecting on surfaces, maintenance, failures, etc. All metal surfaces are bonded to the ship hull so that ground faults can be detected and cleared.

As ships get larger and more electrical power (>6MW) is required, the voltage must increases to decrease current to reduce copper usage. But higher voltage (>1000V) means higher short circuit currents. To limit short circuit current, 6.6kV systems use a high impedance or resistance (1.32kΩ) connected to ground (ship hull) to limit short circuit to 5A. The integrity of maintaining power if a single ground fault occurs is given up because it is to dangerous. Ground faults still occur and may black out the ship, but they must be detected and cleared. The dangers to crew or the ship power or cathodic corrosion protection are more complex.

For the question this means the ship hull is at 0V, but without hull return (no current is meant to flow in the hull).

From IEEE 45.1-2017 - IEEE Recommended Practice for Electrical Installations on Shipboard - Design.

enter image description here

Lower voltages (440V, 120V) are achieved by transformers. At 120V, the loads can be connected as 3-wire delta or 4-wire wye. Individual consumers get one phase and every third light or cabin is a different phase to keep the load balanced as much as possible.

At LV Bus, Neutral is solidly grounded to ship's hull at the transformer. If 4-wire wye (120V/208V) (neutral, 3 phases) is used, neutral is not grounded at receptacles, but power cables may have 3 prongs, Line, Neutral and Ground. So the Receptacle Ground is connected to the hull, so single ground faults in devices plugged in can be detected and cleared.

If 3-wire delta (120V) is used, there is no ground connection at the transformer, but Receptacle Ground is connected to the hull. This is probably where the problem occurs.

enter image description here

USCG show 60V at each terminal, so the system cannot be an insulated neutral. If LV bus is a delta, 60V at each terminal makes sense for a high impedance neutral with hull set at 0V.

120V power strips or surge protectors are designed to interrupt the live L, but there is no protection on neutral N.

Here lies the problem with inexpensive and older SPDs that only disconnect one “hot” terminal lead. The other “hot” terminal remains hot if the circuit breaker supplying the receptacle and SPD does not trip.

All circuits have breakers/fuses on all power lines. The problem occurs when the current is enough to activate surge protectors or power bars, but not enough to trigger built in circuit protection devices.

This fails for either wye or delta, but easier on delta. Delta has three phases, A, B & C. Each outlet gets 2 phases at a voltage of 120V. Cabin 1 has phase AB, 2 has BC, 3 has CA, repeat.

Cabin 1 overloads circuit, power strip breaker trips, but only interrupts A. B is live with 60V (possibly 69V) and because of the plugs ground plug, possibly connects B to ship hull.

Now any other ground fault anywhere on ship occurs and higher than designed currents flow, melting the wire insulation, and causing a fire.

Most likely, fault will occur in another cabin. Hair dryer in Cabin 2 trips power bar circuit breaker, interrupting phase B, but leaving phase C connected and possibly to Ground.

Phase B in Cabin 1 is connected to Phase C in Cabin 2. Short circuit with no circuit protection. Current greater than wires ampacity flows, I2R losses create heat, melt/burn insulation, creating a fire.

The outlet has dual pole breakers connected to a surge protector with a breaker on one side. The problem occurs when a surge occurs activating the surge protector but not meeting the outlet breakers, killing power to one half of the circuit with a potential 60V on the neutral of devices not designed to have a potential on the neutral, causing funky behavior in the wiring, device, surge protector to cause a fire in something probably a device connected to the surge protector.

Properly designed surge suppressors and power strips for ships must have dual pole breakers to disconnect both lines in event of a fault.

  • $\begingroup$ Your diagram shows 60V between ground and live. Does that mean that the ground is also isolated? If so, that means that the receptacle boxes must also be electrically isolated from the ship's physical structure. $\endgroup$ Apr 1, 2019 at 14:25
  • $\begingroup$ Hopefully, that edit clarified your question. If not, ask a new one referencing this. Odds are someone will give you a better answer. $\endgroup$ Apr 1, 2019 at 19:03
  • $\begingroup$ It sounds like it's not so much the electronic surge protection itself that causes problems, but the built-in single-pole circuit breaker (and perhaps on/off switch) that is the culprit. $\endgroup$ Apr 2, 2019 at 12:59
  • $\begingroup$ Surge protector trips circuit breaker, but yes, the problem lies with the single-pole CB. The use of shore based equipment in the marine environment causing the perfect storm. $\endgroup$ Apr 2, 2019 at 14:21
  • $\begingroup$ "A single ground fault (hard or instantaneous) will energize the hull" – It will energize the hull with respect to what? On board a ship, I would expect the word "energized" to mean "at a significant voltage relative to the hull," but if I interpret your sentence that way, it comes out to "a ground fault will cause the hull to be at a significant voltage relative to the hull," which obviously isn't what you're saying. $\endgroup$ Jul 27, 2023 at 18:16

Residential institutions have a history of fatal fires caused by evacuation delays. That's historically true of ships as well as hotels and hospitals. Institutions have two options: periodically check all surge protection circuits, or ban all surge protection devices. Hospitals I've worked in ban all power boards and test everything. Building sites I've worked in have test-and-tag requirements. Passenger ships ban "surge protection".

Anyway, regarding the main question:

The USCG document referenced above concludes with the warning that load disconnection devices should disconnect both legs. This would be true for any site that doesn't have the expected line connection at the socket. That's true for badly wired homes, but for technical reasons is also true of ships.

There seems to be some confusion about what "surge protection" means. If the USCG doesn't recognize a difference between the terms "overload protection" and "surge protection", that is clearly an indication that both passengers and crew will fail to recognize a difference.

So we have several problems:

  • any power board with overload protection should be excluded, regardless of whether it includes surge protection, because you can't tell by looking at it that it disconnects both legs.

  • any power board with surge protection connected to or after the overload circuit should be excluded (but should be anyway), because a voltage surge may trigger load disconnection.

  • any power board with voltage surge protection should be excluded, even if it doesn't include overload protection, because surge protection circuits are designed to protect against transient voltage conditions and then be discarded. This isn't a problem that is "worse" than it is at home, but at home you'll only kill yourself if there is a fire.

  • there are also fault conditions that cause the the neutral and earth to be way off to one side. This is a continuous-fault condition that will cause surge-protection overheating and may cause power-board overheating, failure and fire. This is a fault you are less likely to see at home unless you are multi-phase and independently-earthed. The USCG document referenced in the other answer seems to suggest that ships (like large institutions and factories) are susceptible to this kind of fault: (https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CSNCOE/Safety%20Alerts/USCG%20Marine%20Safety%20Alert%2003-13%20Surge%20Protective%20Devices%20Onboard%20Vessels.pdf?ver=2017-08-08-082206-293)


Many surge suppressors are fire hazards on land and at sea. The energy from a surge is typically absorbed by a MOV. If the surge is long enough, the MOV will get very hot, release smoke, and it can start the surrounding plastic on fire. They were especially bad in the 90s, although more had metal cases which would better contain a fire. More recent ones have thermal disconnects next to the MOV to disconnect it when it overheats. I took one apart to remove the MOV since I dot't like fire hazards and the thermal disconnect was pushed off to the side and not up next to the MOV like it's supposed to be. So even today there are problems. I don't know how these devices were ever UL listed. I've seen one that smoked when the Voltage in a home increased due to a broken neutral.

There doesn't seem to be any reason why power on a ship, whether or not the line is isolated from the ground connection, would increase the risk of a surge suppressor catching on fire. Cruise ships also don't allow electric irons and coffee pots due to the fire hazard.

Most ships have mains that are isolated from the ship's hull (isolated neutral or non existent neutral). But the ground connection is connected to the hull, so the ground is floating in the middle rather than being bonded to the neutral like it is with commercial power. This allows a single short circuit to ground fault to not trip out the power on the ship. It is done for reliability reasons. But equipment on the ship should alert the crew that a ground fault is present. Some surge protectors have a built in ground fault indicator. As someone already mentioned, this would send some current through the ground and could make the ship's short to ground fault detection not work correctly.

The other issue of having an over current trip device on only the hot side of the line, when a ship with isolated mains doesn't have a hot or neutral is an issue. Having both sides of the line trip solves this. But I don't see how there is any way that having an over current trip on one wire is any worse than not having one at all. In USCG document it says "Most commercially available SPDs are designed for use ashore and will interrupt only the hot conductor when a surge occurs." This is completely false and it suggests that the USCG doesn't know what they're talking about. The commercially available SPDs absorb a surge using one or more MOVs. Those MOVs will overheat and catch fire long before the 12 or 15 Amp over current protection device, if equipped, will trip. The over current protection device is there to prevent the cord and receptacle from over heating and reduce the risk of fire. Receptacles may only be rated for 12 Amps, and it is not uncommon for someone to screw a 30 Amp fuse in the fuse box for the circuit. That's a hazard. Even if it is properly fused at 20 Amps, the power strip is not supposed to draw more than 12 Amps out of a single socket. The over current protection has nothing to do with the surge suppression feature. The USCG document also uses the terms Delta and Wye, which are terms used to describe 3 phase power, when describing the voltages on a single phase NEMA 5-15 receptacle. Isolated mains with a floating ground has nothing to do with 3 phase power.

I suspect they want to rock the boat and get people upset about how their "surge protectors" are fire hazards at home as well. That's why the information out there about it states bogus reasons.

USCG source (same is in another answer here): https://www.dco.uscg.mil/Portals/9/DCO%20Documents/5p/CSNCOE/Safety%20Alerts/USCG%20Marine%20Safety%20Alert%2003-13%20Surge%20Protective%20Devices%20Onboard%20Vessels.pdf?ver=2017-08-08-082206-293


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