Creative customisation in the rail industry

Author : Chris Leek, Power Specialist at Charcroft

08 May 2018

While there are standard products that are manufactured to rail-appropriate approvals, some level of creative customisation can also help to ensure fast, effective solutions to failed systems or difficult upgrade projects.

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Failing components and refurbishments are a fact of life for engineers in the rail industry. Dirt and other contaminants, vibration, shock and high transient voltages all contribute to increased risk of failure, and extended system lifetimes can necessitate regular refurbishment projects.

Failing door controls

A typical example of the challenges faced by electronics engineers in the rail industry is the OEM who was experiencing intermittent failure of the door control system on passenger rolling stock. Finding the point of failure was relatively easy – given that some of the PCB-based power supply units (PSU) were catching fire in their door control units.

The cause of the incendiary PSUs was traced to transient voltage spikes, which occurred on the input supply to the PSU, so the solution was to find a replacement unit. With an ultra-wide input range of 33 to 154VDC, the PMDS30 110S24U-VT, from MTM Power, would protect the electronics from transients and provide the equipment with a fully isolated and regulated supply. However, as the new PSU did not match the existing mounting arrangement, a modified, pin-mounted unit was fitted to a PCB with input and output connectors, which matched the existing arrangement.

This slight modification allowed for a quick and simple swap-out of the failing PSU during the normal maintenance programme.

Improving TPWS reliability

In the case of another OEM, a power supply was the cause of reliability problems for their on-board train protection and warning system (TPWS). Although the original unit met the system’s electrical and mechanical specifications, it began to experience reliability problems soon after it went into operation.

Originally, three variants of the open-frame power supply were used to cover the required input voltages of 24V, 72V and 100V. Protection against vibration, water, dust and other potential contaminants was provided by an enclosed metal case, with wires which provided the external connections for the PCB.

The first priority was to address the reliability issue: Charcroft recommended an MTM Power unit which combined patented thermoselective vacuum encapsulation with IP65 sealing.

As an alternative to filling PSUs with resin, the thermoselective vacuum encapsulation process begins with the thorough cleaning of components to remove debris (such as grease or soldering fluxes). This is followed by the drying and pre-heating of the components before encapsulation under vacuum conditions. The result is a solid encapsulation that provides extreme resistance to shock and vibration.

The process also enables the unit to withstand humidity, condensation and high isolation voltages, while also eliminating the need to allow for air and creepage distances during the design process. The encapsulation also helps to support a longer system lifetime by eliminating electrolyte loss from electrolytic capacitors.

The inseparable cemented joint between the potting material and the components means that factors such as ageing, rapid temperature changes, heat, cold and other environmental influences do not result in delamination, cracking or air pockets – all of which can potentially lead to failure.

To increase reliability further still, a secondary circuit, with fully independent isolation and regulation, was fitted to the standard, off-the-shelf unit. This provides a 40V rail for use as a reset, operating independently of the 12V board net. A built-in heat-sink also eliminated the need for external cooling fans which can often be a cause of failure in power supplies.

The new customised supply was fully tested for compliance to EN50155 and EN50121-3-2 by MTM Power before delivery to the OEM.

Higher reliability – lower cost

In addition to overcoming the reliability issues in the TPWS, the PCMDS150WK-IP65 power supply also achieved a 20% reduction in the overall unit cost of each supply. This was partly achieved by the combination of the patented encapsulation process and IP65 rating, which eliminated the hand-assembly which was necessary to protect the original open-frame unit.

The new supply also lowered the overall cost by reducing the number of inventory lines required for manufacturing and maintenance. With the original supply, three different units were used to meet the 24V, 72V and 100V input voltages. With a nominal input range of 24V (16.8V to 33.6V) and 110V (50.4 to 154V), the PCMDS150WK-IP65 was able to cover all three input voltages with just two units.

Isolating HVAC circuit breakers

Even the most flexible configurable circuit breakers may also need some level of customisation to withstand the harsh conditions of the rail environment.

Used in a mix of AC and DC circuits in the heating elements, fan motors and controls for the HVAC system, circuit breakers can be specified for AC or DC operation and configured with a specific trip type (such as instantaneous, slow, long and motor-start delays). The standard AC circuit breakers also provide a barrier between poles 1 and 2, and between poles 2 and 3, to electrically isolate the connections.

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