Insulated Neutral Earthing on Ships
Ships and land based electrical installations differ.
On land based electrical systems, such as those found in homes and factories, the neutral wire (or phase) is connected to the ‘Earth’.
The Earth wire is a safety feature that allows the safe direction of ‘fault currents’ into the ground, and away from people and livestock.
As electricity will travel along the easiest ‘path’, the earth connection provides this.
If a piece of equipment with a metal case did not have an adequate earth, and a fault caused the casing to become ‘live’, then the electricity would travel through a person touching the case.
This is because the human body will conduct electricity (it is very moist). The electricity therefore will use the persons body as a low resistance path between the ‘live wire /or casing’ , and the earth (ground).
Just to be clear about this, what is happening is that the electricity is in effect using the body as a conductor, similar to a wire.
The above scenario is not good for the human body, and a current as low as 30mA (milliAmps) can kill.
That is why electrical installations are designed with safety features such as earth connections, bonding & RCD’s.
Electrical bonding is the connection of metal objects, such as copper water pipes, and gas pipes together.
They are connected via earth straps, and clamped to the metal pipes.
If you take a look behind your bathroom sink, or under your kitchen sink, you should see them (talking about UK installations specifically).
The idea behind connecting the pipes together by wires, is for safety.
This is because the wires are also connected to the earthing system of the building.
If a fault developed, and one of the metal pipes became ‘live’, then there is a low resistance path for the electricity to flow safely away to.
It is important that all metal pipework is ‘ earth bonded’ , due to the following scenario:-
Imagine that the pipe to the cold water tap of a bathroom sink was earth bonded correctly, but not the pipe to the hot water tap.
If an electrical fault occurred and the hot pipe became live, then what is know as a ‘potential difference’ will occur.
Basically the voltage between the two pipes (hot and cold) ,and hence the metal taps, is different.
Therefore if you hold both taps, the electricity will flow through your body, as a path for the electricity.
RCD – Residual Current Device
A Residual Current Device, or RCD is a safety device, which has largely replaced the ‘fuse’ in most current installations.
The RCD detects when the current is at an abnormal level and quickly disconnects the supply. Normally the current flowing in each phase (live & neutral) will be equal, but in a fault
Normally the current flowing in each phase (live & neutral) will be equal, but in a fault condition the RCD will detect an imbalance and disconnect.
An imbalance might be caused by someone touching one of the two phases (wires).
Ships Earthing Systems
Ships earthing systems are typically different from land based installations, when it comes to their Earthing systems.
The system commonly used is known as the ‘Insulated neutral’ system.
As the name implies, the neutral phase wire is totally insulated from (and therefore not earthed to) the ships hull.
On land, the earth connection is ‘tied to’ the neutral connection typically at the local electrical substation, and at the point where it enters the building.
If a fault occurs on a land based installation, the RCD will trip quickly, and cut the supply.
This is ideal for land use, as the priority is to protect human life and livestock.
However, on ship the priority is to keep essential equipment running. For example it would not be acceptable on ship, if an electrical fault caused the Steering Gear to stop working.
For example it would not be acceptable on ship, if an electrical fault caused the ‘Steering Gear’ to stop working, as would be potentially dangerous.
Therefore by using an insulated neutral electrical system, when a fault occurs, the equipment is not disconnected, as would be the case on land.
Instead what happens is that a warning light / alarm will be triggered on the earth fault indicator panel, but the equipment on that circuit will still operate.
Should a second fault occur on the other phase wire, then the safety trip devices would be activated, and the equipment would no longer operate.
It is therefore important to repair a single earth fault as quickly as possible, to ensure safety.
Earth leakage from equipment circuits can also be monitored using the insulation resistance monitor, which will give precise readings of the insulation resistance.
This can be logged over time.
Disclaimer: This article is for information only, and not to be relied upon. If you are unqualified, dont mess with electricity.