Tech advice… Circuit breakers
These small devices are vital for safety
DESIGNERS TAKE INTO consideration two overriding constraints when equipping a caravan with a mains supply system:
Protection from damage and potential fire hazards caused by appliances failing, and thereby exceeding the current rating of the internal mains distribution wiring Protecting the occupants of the caravan from lethal electric shocks in the event of an appliance becoming faulty
Overcurrent protection
Previously, overcurrent (high current demand and/or short circuit) conditions were handled by wire fuse links designed to melt and thereby isolate the supply should a fault situation occur.
Nowadays, households – and caravans – are fitted with current-sensing circuit breakers, known as miniature circuit breakers or MCBS, which isolate the supply once a predetermined steady-state current level has been exceeded or a short circuit occurs.
Type B MCBS (used in houses and caravans) have a stated maximum fixed current rating. This maximum steady-state current (one they can handle continuously) is usually printed on the front of the device preceded by a ‘B’, denoting it is a Type B MCB (for example, ‘B16’ in a 16A device).
The advantage of an MCB is, it operates within quite tight tolerances, tripping out at a point just above its stated current rating – not the case with a wire fuse.
In practice, the current demands on the MCB are varied. With certain loads (mains vacuum cleaners being an example), a large but short-duration transient ‘surge’ current can exist at switch-on before the steadystate current condition is reached. To handle this, MCBS are designed to handle short transient currents of between three and five times their rated value.
In the event of a short circuit occurring, this current level will be exceeded for appreciably longer and the device will trip.
Once a ‘trip’ condition is reached (excess steady current or short circuit), the device reacts rapidly to break the circuit (typically in 30-150 milliseconds). This is much faster than its wire fuse counterpart.
One of the advantages of this type of device is that, once the reason for tripping has been removed, the breaker can be re-engaged by simply pushing a switch lever up on its front. No more looking for fuse wire or a replacement fuse! Using a circuit breaker will also usually keep appliance damage to a minimum, thanks to its fast reaction time, meaning the inevitable ‘bang’, usually associated with a wire fuse blowing, can be avoided.
It should be noted that MCBS can be either single- or double-pole. Single-pole devices are usually placed in the live conductor, so that when an overcurrent condition occurs, the live (high voltage) feed is isolated.
This is the approach taken in the UK domestic environment, which is why it is important to have the polarity correct; if the MCB is placed in the neutral conductor, it will trigger in an overcurrent situation, but the high volts supplied via the live conductor will still be connected to the faulty appliance.
A double-pole MCB breaks both the live and neutral conductors in an overcurrent
situation, thus ensuring that the appliance has no high voltage on it, even if the connections are reversed.
Shock protection
Protection against shock is the job of the leakage current breaker. There are two types: residual current device (RCD) and residual current circuit breaker (RCCB). In essence they are the same.
Power supply basics
Mains power is delivered to the user via the live, neutral and earth conductors.
Live Voltage on this conductor is nominally 230V above the neutral conductor
Neutral Voltage on this conductor is 230V below the live conductor and is connected to the earth by the network supplier Earth This conductor is at ground potential and by default, 230V below the live conductor
Appliances draw their power from the live and neutral conductors; in other words, there is a notional flow of current from the live conductor and back through the neutral conductor.
The earth conductor is usually passive and under normal circumstances, has no current flowing through it at all. It is, however, connected to any metal casing or conductive parts with which the user might come into contact.
According to 19th-century scientist Gustav Kirchhoff’s first law, the sum of currents flowing into a point must equal exactly the sum of currents flowing out of it. This forms the cornerstone of the RCD/RCCB operating principle.
This means that the current going into the appliance/load via the live conductor under normal conditions equals exactly the current being returned via the neutral.
In the event of a fault developing (breakdown of insulation in the appliance or somebody coming into contact with a ‘live’ part), a leakage current to earth occurs.
In this case, the live conductor will be supplying more current than is returning via the neutral, so this will result in an imbalance in the live/neutral currents.
This is detected by the RCD/RCCB, whereupon a breaker is tripped and the supply is isolated. It is generally considered that the human body will be protected if the power supply is rapidly switched off when 30 milliamperes or more of leakage current is detected.
This is why RCD/RCCBS deployed in caravans and domestic properties trip at a 30ma leakage current threshold and, generally, within 40 milliseconds of the leakage being detected.
It is usual to have all of the current that is supplied to a caravan travelling through an RCD/RCCB before being passed to the MCBS, thereby providing blanket shock protection.
All RCD/RCCB devices come equipped with a test button which, when depressed, introduces a simulated leakage current situation, whereupon the trip should operate, if the device is working correctly and is connected to a supply.
The RCBO
It should also be noted that another type of circuit breaker is available, known as a residual current circuit breaker with overcurrent protection (RCBO).
The RCBO combines the properties of an RCD/RCCB and an MCB in one unit, but these are not the norm in caravans.
One of the disadvantages of this type of device is that it trips in the event of a leakage current or a short circuit occurring. This makes finding out which of the two conditions caused the fault in the first place all the more difficult.