What is protection?
The word is defined in the dictionary as a person or thing that prevents someone or something from suffering harm or injury.
In electrical systems that definition can be more narrowly defined as equipment applied to electrical systems to detect abnormal and intolerable conditions and to initiate corrective actions.
Therefore the purpose of protection in electrical systems is to prevent danger to life and damage to equipment, obviously the protection of persons takes priority. Only the circuit supplying a short circuit, overload or earth fault should be isolated by the protective equipment leaving the rest of the system operational.
The protection of persons against electric shock or burns is achieved by insulation or separation and at LV where the risk deems it necessary the implementation of secondary protection by residual current device. The risk to property by fire can be minimised by the incorporation of earth fault protection throughout the distribution network.
Many countries have laws to enforce the requirement for protection. In England that law is the Electricity at Work Regulations 1989 (Statutory Instrument 1989-635). In the design of a system the key clauses of the Regulations that relate to protection are reproduced in their entirety below:-
Regulation 8 - Earthing or other suitable precautions
| Precautions shall be taken, either by earthing or by other suitable means, to prevent danger arising when any conductor (other than a circuit conductor) which may reasonably foreseeably become charged as a result of either the use of a system, or a fault in a system, becomes so charged; and, for the purposes of ensuring compliance with this regulation, a conductor shall be regarded as earthed when it is connected to the general mass of earth by conductors of sufficient strength and current-carrying capability to discharge electrical energy to earth. |
Regulation 11 - Means for protecting from excess of current
| Efficient means, suitably located, shall be provided for protecting from excess of current every part of a system as may be necessary to prevent danger. |
These two regulations are the basis for the legal requirements for protection. It is also logical to provide protection to protect the financial investment that has been made in the electrical system and the equipment fed by that system.
The Electricity at Work Regulations can be downloaded from the UK government’s website at the following address:-
http://www.legislation.gov.uk/uksi/1989/635/contents/made
the guidance notes to the regulations (the HSE guidance document HSR25 translates each of the regulations from incomprehensible legalese English into normal English) can be downloaded from the Health and Safety Executive website at the following address:-
http://www.hse.gov.uk/pubns/books/hsr25.htm
A transformer that is destroyed by a fault takes a long time to replace, the consequential financial losses can be significant which must be added to the replacement costs of the transformer. Therefore proper and proportionate protection not only allows compliance with statute, it is also both logically and financially prudent.
In its most basic the protection must protect the load and its supply cables from short circuit, overload and earth fault.
A short circuit is where a direct phase to phase, phase to neutral or phase to earth fault occurs, this type of fault places the highest stresses on the equipment and must be detected quickly and isolated from the electricity supply.
To protect the system from short circuits a fuse, circuit breaker with integrated protection or a circuit breaker with a separate protection relay may be used.
A fuse is a sacrificial device that by its very design restricts the level of fault current that can flow through it as it ruptures and becomes open circuit before the full available fault current flows.
A circuit breaker with integrated short circuit protection may use a number of methods to detect and open the circuit breaker, high short circuit levels may be detected by mechanical/magnetic means and the circuit breaker forced open typically in less than 10milliseconds or electrically when the operating time will be longer or a combination of electrical and mechanical protection.
A circuit breaker with a separate protection relay uses current transformers to determine the level of current flowing in a circuit when that current exceeds a preset limit for a preset time the relay operates and signals to the circuit breaker to open the circuit and isolate the supply, the time taken for the circuit to be isolated will be a minimum of the relay operating time plus the circuit breaker opening time which will normally be less than 50milliseconds.
An overload is where the current in a phase or neutral exceeds the rated current of the equipment being supplied or the cables supplying the equipment.
To protect the system from overcurrents a fuse, circuit breaker with integrated protection, circuit breaker with a separate protection relay or a contactor with separate protection relay may be used.
Fuse elements are designed to burn away and isolate the load and its supply cables in accordance with a time current curve that meets the standards to which it was built. In England the prevailing standard for fuses is British Standard 88 part 2 which equates to EN 60269 part 2 and IEC 60269 part 2.
A circuit breaker with integrated overcurrent protection detects and opens the circuit breaker according to the time current settings of the protection element of the circuit breaker. The relay curve characteristics of the circuit breaker with integrated protection are dependant on the manufacturer of the circuit breaker, the characteristics tend not to follow mathematical formulae.
A circuit breaker with a separate protection relay uses current transformers to determine the level of current flowing in a circuit when that current exceeds the current and time settings of the relay’s pre-set characteristic curve the protection relay operates and signals to the circuit breaker to open the circuit and isolate the supply. The relay curve characteristics are laid down in BSEN 60255 and follow mathematical formulae.
The equation for the standard inverse curve is t=0.14/(I0.02-1)
The equation for the very inverse curve is t=13.5/(I-1)
The equation for the extremely inverse curve is t=80/(I2-1)
where t=time in seconds and I=current in amps
The inclusion of a contactor and separate overload relay is included for completeness only. That particular combination is normally used with a fuse to back up the contactor, the overload detection unit has a time current curve that compliments the fuse curve in such a way that the fuse responds to short circuits whilst the overload unit responds to overloads. It is not usual to see this type of protection applied to distribution systems, however, it is quite common to see it applied to motor starters.
An earth fault is where current flows from a phase or neutral to earth. Due to inductive and capacitive coupling every circuit will have some leakage to earth, that leakage is normal and should not cause the protection system to operate.
To protect the system from earth faults a fuse, circuit breaker with integrated protection, circuit breaker with a separate protection relay or a contactor with separate protection relay may be used.
A fuse will require the same level of current to earth to cause it to burn away and isolate the load and its supply cables as that required for short circuit and overload currents.
A circuit breaker with integrated overcurrent protection, but without dedicated earth fault protection detects and opens the circuit breaker according to the time current settings of the protection element of the circuit breaker utilising the same current levels as the short circuit and overload protection.
A circuit breaker with integrated overcurrent and earth fault protection detects and opens the circuit breaker according to the time current settings of the earth fault protection element of the circuit breaker, the current and time settings will be specific for earth fault protection.
A circuit breaker with a separate protection relay uses current transformers to determine the level of current flowing to earth when that current exceeds the current and time settings of the relay’s pre-set characteristic curve the protection relay operates and signals to the circuit breaker to open the circuit and isolate the supply. The earth fault settings are much lower than the overcurrent settings.
The inclusion of a contactor and separate overload relay is included for completeness only. That particular combination is normally used with a fuse to back up the contactor, the earth fault detection unit has a time current curve that compliments the fuse curve in such a way that the fuse responds to short circuits whilst the earth fault unit responds to earth faults. As with overloads it is not usual to see this type of protection applied to distribution systems, but it is quite common to see it applied to motor starters.
Overloads can result in the heating of the equipment incorporated into an electrical system typical of these are transformers and motors where heating can have detrimental effects on the equipment’s insulation; which can in turn lead to an insulation breakdown. That over heating can be detected by probes placed in the windings of the equipment. The operation of the over temperature device removes either the power to the equipment or removes the load. It is important that equipment that contains cooling fans is not isolated from the supply, if that supply powers the cooling fans, as the fans are required to cool the overheating equipment which is why it is the load that should be isolated and not the supply.
The protection described in the following parts of this section gives details of the protection utilised in primary distribution systems.