Circuit-breaker applications

Circuit-breaker applications – Simple solutions with the latest Design software

Introduction

Designers of today’s electrical distribution systems have available to them a bewildering choice of circuit protective devices. These range in sophistication from simple fuses and miniature circuit-breakers (mcbs) with fixed tripping characteristics to microprocessor devices that are available for use as separate relays or incorporated into circuit-breakers. Designers can select from readily available products with functions and settings that can satisfy almost any protection requirement.Along with this wealth of choice comes the considerable challenge of selecting the right device for each situation and ensuring that all devices chosen are correctly set to provide adequate system protection.The various advantages and disadvantages of the use of fuses and circuit-breakers for distribution protection are not considered in this article. Suffice it to say that under certain circumstances fuses will be seen as an ideal choice, whereas for other conditions, circuit-breakers may be considered as the best solution. Any designer having to deal with distribution systems intended for a wide variety of applications will, inevitably, have to deal with the challenges of both fuse and circuit-breaker application, often mixed in the same installation.Where fuses are an adequate solution to system protection, their selection to ensure all circuits in the distribution system are adequately protected and that acceptable levels of discrimination are maintained, may be a relatively straightforward task. However, using even relatively simple circuit-breakers may considerably increase the design work necessary, while the application of sophisticated relay and circuit-breaker protection can present a considerable challenge.So, the good news is that there are many circuit-breaker devices available that can potentially solve almost any protection problem a designer may be faced with. As ever, the bad news is that there is more to consider, more to go wrong!Fortunately help is at hand. Designers have for many years used design software to assist in distribution system design tasks such as: load current calculation, cable sizing, voltage drop calculation, etc. Such software is now available with sophisticated tools that allow designers, using almost any of the currently available circuit-breaker products, to apply any protection settings and then evaluate the effect of any resulting changes to the tripping characteristic of the device on network cable sizing calculations and discrimination. This article looks at some of the various design challenges associated with circuit-breaker application and gives examples of areas where such design software can be of assistance.

Distribution system protection requirementsSummarised below are some of the design issues specifically related to circuit-breaker protection for which software can be useful.

Interrupting capacity: it is essential that the designer is alerted to any instance where the prospective short-circuit fault current (Pscc) exceeds the interrupting capacity (Icu) of the circuit-breaker installed at that point

Overload, short-circuit and earth fault protection conditions: checks must be made to ensure that the circuit-breaker and its settings are selected to provide the circuit with protection against hazards associated with overload conditions

Discrimination: where required for safety and convenience, it is essential to verify that discrimination will be maintained between successive circuit-breakers under all anticipated fault conditions

There are also a host of other design problems not related to circuit-breaker applications, and therefore not discussed here, for which distribution system design software can offer assistance. These include: cable and busbar sizing, voltage drop assessment, phase and earth fault calculations, diversity, etc.

Overload ProtectionFor any circuit, the current rating (In) of the selected circuit protective device should be equal to or greater than the design current (Ib) for that circuit: 

For a fuse or non-adjustable circuit-breaker, this is easily checked. For an adjustable circuit-breaker, it is necessary to know what settings are available and how they are applied. For example, it is not uncommon for the electronic protection on circuit-breakers to have two overload current settings, the resultant overload setting being the product of the two. Software that holds all the information for the required circuit-breaker, including all the protection settings, how they interact, what the resulting tripping characteristic looks like, can be used to select the correct setting with complete accuracy and confidence, without reference to the manufacturer’s data or other investigation.

SOFTWARE APPLICATION EXAMPLE 1:Selecting a circuit-breaker and making the correct settings for a circuit with a design current (Ib) of 350 A.

1. Select the required circuit-breaker with a rating In = 400 A.2. Click button to switch to graph view where the settings can be made and the characteristic curve can be viewed.3. Overload current settings initially at: Io = 1; Ir = 1. Overload setting = 1 x 1 x 400 = 400 A.4. Adjust settings to nearest overload setting to circuit Ib. Overload setting = 0.90 x 0.98 = 352.8 A. ‘Tool tip’ shows current setting in amperes for selected setting combination.

5. The displayed characteristic is updated for each setting change, with the value of the last changed setting indicated.

 

Short-circuit protectionOften when setting short-circuit protection, unless consideration is given to all factors, it is all too easy to end up with current settings adjusted either excessively high or low. If an unduly low setting is made this can result in nuisance tripping when inductive loads such as motors, transformers, etc. are energised.At the other extreme, making high short-circuit settings leads to a number of potential dangers. Firstly, as a general principle, it is not good practice to make short-circuit settings higher than they need to be. During a short-circuit the thermal effects associated with the high level of current can cause damage to conductors and other circuit components. Secondly, if the short-circuit current trip setting level is in excess of the Pscc the circuit will effectively be left with no short-circuit protection.Finally, even if excessively high or low settings are avoided, it may still be necessary to check the

Earth fault protectionOne advantage of using circuit-breakers is models that include residual current earth fault protection are readily available. This is a feature that is particularly useful in situations where the earth fault current (Ief) is low and consequently, compliant earth fault disconnection is difficult to achieve using overcurrent protection.

DiscriminationOvercurrent protective devices should be set so that they discriminate, where necessary to prevent danger (BS 7671: 533-01-06).Discrimination is achieved where, under fault conditions, the protective device nearest the fault operates rather than any protective device upstream of it. This ensures that a fault will be disconnected with minimal disruption to any other circuits in the system.When verifying discrimination between any two circuit-breakers, it is necessary to view their tripping characteristics to ensure that, up to the Pscc at the downstream device, there are no points at which the two characteristics overlap. Such overlaps indicate levels of fault current that may cause the upstream circuit-breaker to operate before the downstream device has time to clear the fault, i.e., loss of discrimination.Additionally, for fast-acting current-limiting circuit-breakers, it may also be necessary to consult the manufacturer’s discrimination tables to verify discrimination for heavy fault currents where the circuit-breakers are operating rapidly in current-limiting mode.For an extensive distribution system, the necessary investigations when performed by hand, would be very time consuming. Again the use of software offers a great time-saving potential. A circuit-breaker with a higher current rating is used in the upstream circuit. The network design is re-calculated to confirm that the problem has been corrected.

SOFTWARE APPLICATION EXAMPLE 2Producing a discrimination study as part of the documentation for a distribution system design.Where, as is often the case, a discrimination study or a number of studies are required as part of the design documentation, the same software can be used to assist in the design of the network and to produce professional discrimination studies in a fraction of the time required when doing it by hand.1. On completion of the distribution system design, select a circuit (small area of total drawing shown).2. Click discrimination study button.3. Discrimination study including graph is produced.

The above example discussed above relates to phase fault discrimination but equal care should be taken of discrimination between residual current devices, (BS 7671: 531-02-09).

ConclusionWhen faced with the extra design work associated with the use of circuit-breakers, it would be all too easy to always opt for fuse protection. While this may, in some cases be the appropriate course of action, there are applications where circuit-breakers can offer benefits that should not be ignored, and in fact their use may be demanded by the client. In such cases electrical design software offers a way of getting the design completed on time and, at the same time, ensuring that essential checks are not overlooked.

Author: Roy HughesRoy Hughes is the Senior Electrical Engineer at AMTECH Power Software. Roy has a wealth of knowledge gained from over 30 years experience within the industrial and marine industries.