Tuesday 26 January 2010

When is the installation of RCD protection required?

17th edition regulations - circuits requiring RCD protection

The following three items have been affected in terms of RCD requirements by the changes in the 17th edition regulations, these are

1. Socket outlets
2. Cables buried in walls
3. Locations containing baths and showers

Socket outlets are affected by the regulation 411.3.3 which states, in general terms, that all socket outlets up to 20A must have RCD protection and that sockets up to 32A which may be used for outside use must also be RCD protected. The above two items are only required when the outlet in question is to be used by ‘ordinary persons’ i.e. home owners, office workers, cleaners, ect.

The requirements for the installation of RCD protection shown above exceptions may be made where, the use of socket outlets is supervised by ‘skilled’ or ‘instructed’ persons or if the socket outlet is suitability identified or labelled for a particular item of equipment.

All cables buried in walls now require RCD protection assuming none of the following items have been met

1. The buried cable incorporates an earthed metal covering which is suitable as a protective conductor i.e. SWA cabling.
2. The cable is fully enclosed in an earthed metallic conduit which is suitable as a protective conductor.
3. The cable is fully enclosed in an earthed metallic trunking which is suitable as a protective conductor.
4. The cable being protected against damage by penetration by nails or screws. NOTE: this requires the cable to be buried to a depth of at least 50mm in all directions.
5. Be installed in a safe zone.

This is very similar to the sixteenth edition regulations the normal course of action would have been to install the cable in a safe zone, the fifth point, but the 17th edition regulations implements the following stipulation

Where the use of the installation is not supervised by ‘skilled’ or ‘instructed’ person regulation 522.6.7 applies, this states that RCD protection not exceeding 30mA is required.

Overall these regulations require that RCD protection will be required on all domestic socket outlets using thermoplastic cabling, and that RCD protection will also be required in many commercial situations. Below is a selection of pictorial examples of RCD protection requirements

Sourced from: Hagar

In locations containing a bath or a shower, section 701, the requirements for RCD protections have also been changed. Regulation 701.411.3.3 states that all circuits within this area will require RCD protection not exceeding 30mA. This includes fans, showers, lighting, electrical heating devices, ect.

Further more to this 13A socket outlets are now acceptable, provided the outlet is at least 3m from the boundary of zone one.

Within the sixteenth edition regulations required; local supplementary bonding be provided connecting together all exposed and extraneous conductive parts in the (bathroom) zones. This is now not required assuming all of the following items are met

1. All final circuits in the area comply with the automatic disconnection requirements of regulation 411.3.2
2. All circuits are RCD protected by regulation 701.411.3.3
3. All extraneous-conductive parts of the location are effectively connected to the protective equipotential bonding according to regulation 411.3.1.2

As well as the three above conditions RCD protection is also to be recommended for any outside installation. It may also have to be considered for a cooker circuit containing a socket outlet by the regulations mentioned above (it is a commonly forgotten outlet).

Construction of AC cage and wound rotor motors

Squirrel cage motors

The majoritory of 3-phase AC induction motors are of the squirrel cage construction. They are constructed in two parts, a fixed outer portion, called the stator and an inner portion which rotates inside the stator, this is called the rotor. Between the two there is a carefully engineered air gap.

In a 3-phase induction motor a rotating magnetic field is created naturally within the stator, this is due to the nature of the 3-phase supply. Two sets of electromagnets are created within the motor, the first is formed in the stator, and this is due to the 3-phase AC supply to the stator windings. The alternating nature of the supply induces an EMF in the rotor by Lenz’s law, this action generates another electromagnet in the rotor. The interaction between these fields creates a twisting force or torque. This results in the rotating of the rotor in the direction of the torque.

In a single phase induction motor the rotating magnetic field in the stator in not naturally occurring, in this case the motor has to be started by inducing two supplies of differing phases from the single phase supply. When the motor is beginning to run at operating speeds a centrifugal switch cut out the starting circuit and will return the supply to single phase alone. Below shows one of many starting technique for a single phase AC induction motor, capacitor start capacitor run starting.

All about circuits. (2009) Single-phase induction motors.
http://www.allaboutcircuits.com/vol_2/chpt_13/9.html. Accessed 29/11/09.

The speed of the motor is dependent on the slip between the two rotating magnetic fields, if the stator magnetic rotation exceeds that of the rotor the motor will act as a motor, if this is reversed and rotor field exceeds the stator field then the motor will act as a generator. If there was to be zero slip i.e. the fields are rotating in unison then the windings become a transformer, this state is inadvisable and can cause damage to the motor. Speed is also a function of the frequency of the supply and by the number of poles contained within the motor, the speed if an induction motor can be shown as


Ns= The synchronous speed of the stator magnetic field in RPM
f= The frequency of the supply
P= Number of poles within the motor

The stator of an 3-phase induction motor is constructed as below

Balu, S. (2009) How Are Squirrel Cage Induction Motors Constructed?. http://www.brighthub.com/engineering/electrical/articles/43723.aspx#ixzz0Zmyhl8jY. Accessed 10/12/09.

Though not shown on the diagram above, the stator is commonly made up of thin insulated laminations of aluminium or cast iron; these thin laminations are preferred to a solid mass as they reduce eddy currents produced in the casing. They are either punched or clamped together forming a hollow ring, known as the stator core. It can be seen above that the stator is constructed with channels running through the inside of the core, the insulated stator windings are installed in these slots. Slotting is used in both the rotor and the stator this improves the motor by,

  • Reduces the size of motors by giving the windings somewhere to be run, without increasing the overall bulkiness of the construction.
  • It ‘focuses’ the lines of flux created in the machine, allowing less magnetic losses.
    The slotting reduces the magnetic reluctance of the machine.
  • The lamination of the core and increase in surface area by slotting also reduces the eddy current losses in the machine.

Each opposing phase winding creates a pair of poles (or an electromagnet) on the application of the AC supply. The above diagram shows a machine with six poles. The three phase windings are placed on the slots of laminated core and these windings are electrically spaced 120 degrees apart. These windings can be connected as either star or delta depending upon the motor requirements. The leads are taken out usually three in number, though more or less are possible, and brought to the terminal box on the external of the motor frame. The stator is constructed in this way for most of the AC induction motor family.

A squirrel cage rotor, is constructed as shown in the diagram below

Knowledge publications. (n.d.) No title. http://knowledgepublications.com/doe/images/DOE_Electrical_Science_Squirrel-Cage_Induction_Rotor.gif. Accessed 1/12/09.

A squirrel cage rotor consists of, as with the stator, a number of thin laminated cores (shown well in the top diagram) which when constructed gives a number of skewed slots which contain the conductors, which are not wires, in the conventional sense, but thick, heavy bars made of copper or aluminium or similar alloys. The conductors are evenly spaced, are solid with no joins and are connected mechanically and electrically by the copper end rings (as shown in the diagram). The end rings are commonly welded, electrically braced or bolted at both ends of the rotor, thus maintaining electrical continuity.

One important point to be noted is that the end rings and the rotor conducting bars are permanently short-circuited, thus it is not possible to add any external resistance in series with the rotor circuit for starting purpose.

The squirrel cage design is robust and reliable, this is the reason why they are so wide spread, the reason for these positive points are due to the skewed nature of the rotor, and this is an advantage because

  • It makes the motor run in a less noisy way, by reducing the magnetic hum of the machine.
    It decreases the slot harmonics
  • It helps reduce the locking tendencies of the motor. Rotor teeth may become locked with the stator teeth when direct magnetic attraction occurs between the two. This only occurs when the number of stator teeth is equal to those in the rotor. With the skewed formation on the rotor this is many less likely.
  • Increase in effective ratio of transformation between stator & rotor,
  • Increased rotor resistance due to comparatively lengthier rotor conductor bars,
  • Increased slip for a given torque.

The rotor is mounted on a shaft using bearings on either end to allow rotation occur. One end of the shaft is normally lengthier to allow for a load to be applied. Some motors may also have an ancillary shaft at the non load end allowing the mounting of speed or positioning devices. An air gap is still maintained allowing the induction between the two parts to occur. Regardless of the rotor employed the principle of rotation is the same.

Squirrel cage motors can have heating issues at low speeds, this is due to the fact that the fan is fitted to the end of the rotor and is only able to rotate at the same speed, the cooling capability of the motor is proportional to the rotor speed, thus at low speed the motor is unable to cool itself and overheating can occur.

Another common problem with cage motors occur when high inertia loads are being driven. The start up current is very high and only falls when the reverse EMF is created in the stator through inductive nature of the motor, as previouly mentioned. If the start up time is prolonged due to having tto start a high inertia load then the current will remain higher for a longer period, this causes greater heating in the windings, and thus increases the potential for damage to them, it also causes more mechanical stress on the machines parts and in turn lowers the motors life expectancy.

The wound rotor motor

Tpub. (2009) No title. http://www.tpub.com/content/doe/h1011v4/img/h1011v4_32_1.jpg. accessed 04/09/09.

The wound rotor motor is a variant of the squirrel cage motor arrangement. The stator of a wound rotor motor is constructed and operates in the same way as the squirrel cage motor. The rotor conductors, though, are not short circuited but are terminated to a set of slip rings (as in the diagram above), these slip rings do not have power applied to them, and they are there purely for application of resistances. This is a help in adding external resistors (which is impossible in a squirrel cage rotor) to reduce currents drawn in the starting of the motor.

The slip required to generate a maximum torque, is directly proportional to the resistance of the rotor. In the wound rotor motor, external resistors can be installed; this increases the rotor resistance, which in turn means it is possible to increase the slip of the motor. If the slip is increased then the maximum torque will be available at a lower speed. If a particularly high resistance is applied to the rotor then the maximum torque maybe attained at almost zero speed, this provides a high torque with a very small starting current.

As the motor is accelerated the resistance maybe adjusted to suit the load requirements. When the motor reaches the required speed then the resistance is removed completely and the motor operates as a standard induction motor.

The wound rotor motor is perfect for driving high inertia loading, in this situation it is required that maximum torque is applied at the lowest possible speed in order to start the load rotating as the speed increases the torque required is lowered and so the resistance is reduced and at full speed is removed fully.

Electrical Resource. (2009) wound rotor motor construction
http://www.electrical-res.com/wound-rotor-motor-construction/. Accessed 09/12/09.

The construction of the wound rotor differs to that of the squirrel cage motor because instead of the rotor bars it contains three windings similar to the stator. The windings, which are open, are connected to three slip rings mounted on the rotor shaft (as seen above). This rotor circuit is connected to three brushes to an external star connected variable resistance as mentioned before in the operation section.

The main disadvantage of the wound rotor motor is its requirement for maintenance to the slip rings and brushes, as squirrel cage does not require this. It has advantages in the ability to drive high inertia loads, controllable start up currents, speed can be controlled (by resistance) from 50-100% of full speed.

DC injection breaking

DC injection is the most basic and widely used form of AC motor braking, this is due to its effectiveness, efficiency and simplicity. In this process a DC current is applied to the fixed stator windings in the AC motor, via two or three of the terminals after the three phase AC current has been removed, via a contactor. The DC supply is obtained from a rectifier fed via a low voltage high current transformer; it is also simple to gain when using variable speed drives using PWM, as the PWM can feed DC pulses to the motor terminals.

This causes the magnetic field of the stator to be of a fixed, as opposed to the rotating magnetic field of the 3-phase AC supply, as the speed of rotation of the air-gap field is directly proportional to the stator frequency, it should be clear that since DC is effectively zero frequency (as it is linear wave form), the air gap field will be stationary, and it is known that the rotor of a AC motor will always try to rotate at the same speed as the air gap field, if this is zero then the rotor will in turn try to become stationary. when the poles of the stator are alined (N-S and S-N) the rotation of the rotor will stop, the only thing that will over come this stopping power is the inertia of the rotor and its load.

The overall effect is that the motor will stop quickly and smoothly, the rotor will be held in place and resistant to any external force for the period that the DC voltage is applied. The strength of the braking force is controlled by the magnitude of DC voltage applied, the higher it is the stronger the braking force will be. The braking characteristic for a AC cage motor is shown below

Tinamics. (2009) DC injection braking. http://www.tinamics.com/s0103/index.php?tpid=0062&pgid=0010&menusub=12. Accessed 3/1/10.

The above graphic shows that the reverse troque falls to zero as speed reduces to stand still. It can be also be said that the speed torque braking characteristic is a morrior image of the motoring speed torque curve.

If the DC voltage is kept on the stator the rotor will be held in place, but if DC injection is continued over a prolonged preiod of time it can cause damage to the motor. Although the DC injection technique can be used for any load or torque level it is prefred that a smaller load is breaked as the larger the load the more detremental the braking system is on the motor.

DC injection is a standard feature of AC drives today. If the DC injection was not applied the motor would slow when the feed is removed but would take a long period of time to stop and may not be acceptable under emergency conditions or for use with potentially dangerous machinery. The machanical maintainace is not increased as the braking is provided without a need for touching parts.

A DC injection break in a 3 phase motor system would be installed as per the depiction in the graphic below.

By Ian Loram-Martin, using CAD 2007.

The applications of the DC injection break are many; one example of this is in use with synchronous wind turbine generators. When wind turbines are under great stress from excessively quick winds, the propellers will attempt to rotate at a rate which will damage the turbine structure, eventually the turbine could rip its self apart. In order to slow the rotation of the propeller blades a DC injection break can be implemented as shown in the following diagram

Law, G. (n.d.) FdSc_-_Lect_Sync_Generator_Wiring. http://moodle.cityplym.ac.uk. Accessed 1/1/10

The above diagram shows, from right to left, the propeller and synchronous generator setup is shown. The AC energy from the generator is converted to a DC form via the diode rectifier. The high frequency interference is dampened by the inductor giving a cleaner DC supply. The DC power is stored within the capacitor which in turn feeds the inverter allowing for control of the AC output in terms of voltage and frequency. The central SCR Blocks the DC voltage from being fed back into the generator, if this device was to be switched on a DC injection would be fed into the stator of the generator slowing its rotation, the level of voltage, controllable with PWM, determines the strength of the braking. Using this process the propeller rotational speed will be slowed or even stopped and held in position in high wind conditions.

DC injection braking has the following disadvantages in terms of energy efficiency
• Losses through heat when used for a prolonged period of time or if it is used with an existing supply present.
• Requires, in some cases, a separate DC supply. This produced DC energy is all lost through the braking process and heat.
• Losses in rectification process from the transition from AC to DC, if the AC supply is used to create the DC voltage.
• Mechanical energy is lost, by heat, by stopping the machine at a greater rate.