Thought leadership White Paper On board with Eaton. Designing hydraulic-magnetic circuit breakers for hazardous rail applications White Paper WP131005EN Effective January 2017 Author name The nature of the rail challenge Alexandre Zint, Conditions on board a train are harsh for equipment mechanically, Product manager, Eaton electrically and environmentally. Additionally, the equipment is expected to endure these conditions throughout near-continuous operation over very long lifetimes. The rail industry is highly competitive, and rail operators Due to space limitations, the electrical equipment is usually tightly are under pressure to make their trains safer, more reliable, packed into small enclosures, cabinets or compartments, with units cost-effective and attractive to travellers. Choosing hydraulic operating in very close proximity to one another. This can increase magnetic circuit breakers for circuit protection can help in the incidence of spikes, transients and bursts in the power lines meeting these objectives, as they perform reliably under that circuit breakers have to protect. Within applications such as the harsh conditions of a rail environment while eliminating metros and tramways, track distances and time intervals between nuisance tripping and its associated problems. In this White stops are short, with multiple journey breaks and traction step Paper Jean-Christophe BARNAS, senior engineer at Eaton changes. These, together with heavy passenger door usage, explains the technology and details the advantages that cause hard repetitive switching which subjects the equipment and it offers. circuit breakers to constant voltage fluctuations. Additionally, there are often long wiring runs between the circuit breakers and the Railways have long been recognised as particularly harsh equipment they protect, because the circuit breakers are usually environments, placing extreme demands on the equipment assembled onto panels within the driver’s cab or into electrical installed into their rolling stock and trackside applications. However, cabinets located in the coaches. Alternatively they may be housed simply meeting these challenges, though essential, within low voltage boxes on the carriage’s underframe or even on isn’t sufficient – while doing so the equipment must also play its roof. its part in helping operators’ drive for an ever safer, more reliable, attractive and cost effective environment. They work under Sudden, fast temperature fluctuations can occur during a journey intense competition where global safety standards are as the train enters different geographies or goes through tunnels. increasingly stringent and railways are just one transportation These can affect equipment operation and may impact their option for travellers. electrical and/or mechanical performance. Current fluctuations in the electrical circuits can arise as a direct result. The circuit Electrical equipment and circuits are particularly important in these breakers can also be subjected to hard mechanical shock, considerations. Protecting them is critical to ensure passenger and vibrations, accelerations and jerking movements transmitted staff safety and minimise downtime, costs and delays arising from directly from the body shell of the train. The severity of these will any malfunction or failure. Eaton’s line of hydraulic-magnetic circuit depend on the location of the circuit breaker panels. The potential breakers offers operators a powerful tool for improving circuit impact of these environmental issues can be summarised as protection on board trains. In this paper we look at why these follows: devices are especially resilient in rail operating conditions, and why they offer a more reliable, trouble-free, safe and cost effective • Breakers may be subjected to extreme electrical and solution than thermo-magnetic circuit breakers. physical disturbances • Circuit protection may not be optimised to actual power supply and load requirements • There are several factors that could lead to nuisance triggering • In the worst case, protection provided may not be effective 2 EATON – Precision power protection and shock buffering in a rail environment White Paper WP131005EN Effective January 2017 The hydraulic-magnetic circuit An overload current causes the bimetallic element to heat up and deform, thus opening the contacts and breaking the power circuit. breaker solution The bimetal has thermal inertia, which provides a useful time Hydraulic-magnetic circuit breakers lend themselves naturally delay for overload transients, but renders it too slow to safely limit to solving these issues, and accordingly they have become well short circuit energy. Therefore, the thermo-magnetic breakers also established in the sector after being introduced by the different have an electro-magnetic system which quickly trips the breaker rail manufacturers. They provide a better circuit protection solution on short circuit or high current levels; a reaction within 2 ms is than thermo-magnetic devices for the rail environment. Overall, possible. Electro-magnetic tripping generally occurs at six times the they can be seen as energy buffers that absorb abrupt and sudden circuit breaker’s nominal current. mechanical and magnetic variations. Additionally, as we shall see, However these devices have a couple of major weaknesses. many detailed advantages arise from both the functional and First is their dependence on ambient temperature which leads physical aspects of their design. to significant variations – of up to 50% - of the tripping point. Taking To understand why hydraulic-magnetic circuit breakers are this temperature parameter into account when selecting the circuit functionally so much better-suited to the rail environment and its breaker’s nominal current is absolutely mandatory. challenges than the alternative thermal-magnetic approach, we can Secondly, compared with hydraulic-magnetic circuit breakers, start by reviewing the operating and fault conditions that any circuit the thermo-magnetic devices might have relatively high internal breaker must handle, and then compare the hydraulic magnetic and resistance; this generates a voltage drop across their terminals. thermal-magnetic solutions to these conditions. Switching on capacitive loads may immediately trip a sensitive During normal operation, the current drawn by the load through breaker, making it impossible to start the equipment. its protecting circuit breaker is within the breaker’s nominal current By contrast, hydraulic-magnetic circuit breakers deliver precision limits, and under these conditions the device will allow current to protection and a controlled response to changes in load current. flow indefinitely. In most installations, however, an overload current They provide reliable, consistent protection characteristics in all is likely to arise at some point, either because a fault condition has environments. Fig. 1 below shows their operating principle. A developed, or, more usually, because equipment such as a motor load current is passed through a coil of wire wound around a or transformer has been turned on. Starting inductive equipment hermetically-sealed non-magnetic tube containing a spring-loaded, such as this draws a heavy inrush current, creating a power surge moveable iron core and silicone oil fill. With the load current through the supply circuit and circuit breaker. either at or below the breaker’s nominal rating, the magnetic flux Because these surges represent transient overloads, they usually produced lacks the strength to move the core, which remains at pose no threat of damage to the line or equipment. Therefore, the end of the tube furthest from the armature. interrupting the supply when they occur is not necessary or even On overload, the magnetic flux force increases, pulling the iron desirable. In fact, such interruptions are referred to as ‘nuisance core into the coil and towards the pole piece and attracting the tripping’. Nevertheless, because the overload current may be armature. The silicone oil regulates the core’s speed of travel, caused by a genuine fault condition rather than a start-up transient, creating a controlled trip delay that is inversely proportional to power must be interrupted if it persists for too long the heavier the the magnitude of the overload. If the overload current subsides current, the shorter the allowable trip time delay. before the core reaches the pole piece, the spring returns the In more extreme circumstances, a fault may cause or may be core to its original position and the breaker does not trip. However seen as a short-circuit current. This is much heavier than an if the magnetic flux reaches a predetermined value, the armature is overload, with the potential to damage equipment or injure people attracted to the pole piece and the breaker trips. The breaker may very quickly. Any circuit breaker must open as fast as possible to even trip before the core reaches the pole piece if the critical flux minimise the energy it lets through. value is reached first. On very heavy overloads, or short circuits, the flux produced is 12 374 8 sufficient to pull in the armature regardless of the core position. This provides immediate circuit interruption with no intentional delay – a very desirable characteristic. Fig. 2 compares the response curves of the two technologies and shows how a proportion of the thermo-magnetic curve is temperature dependent. 5 6 Temprerature independance Circuit breaker with no load Circuit breaker severely overloaded Temprerature