The Railway Technical Website

This is a simplified description of the air brake control system known as PBL90.  It is based on the UIC standard and variations of it are now used on some trains in the UK. It is assumed that if you have chosen this page, you probably have a good understanding of train braking systems in general and of air brakes in particular. 

Introduction

The PBL 90 electro-pneumatic system is a Brake Control Panel (type PBL 90) mounted inside the equipment compartment, usually on the brake frame. The Brake Control Panel performs the function of the driver's brake valve on a conventional British locomotive and is standard equipment on the SNCF and SNCB.  In French it is known as the "Robinet de Mechanicien" (driver's brake valve).

The principle of operation of the Brake Control Panel is that it transfers the driver's braking commands (or those of an ATO system should it ever be fitted), which originate as electrical signals, into pneumatic signals to control the pressure in the brake pipe. It also transmits the signals electrically along the train to provide electro-pneumatically assisted brake control on each vehicle.

The Brake Control Panel is also designed to respond automatically to unplanned or emergency losses of brake pipe pressure to ensure that such incidents cause the train to stop as quickly as possible.

In order to simplify understanding of the Brake Control Panel, this article provides a series of simplified schematics which show the panel with only its principal parts. 

The Parts of the Brake Control Panel

Figure 1: This shows a simplified schematic of the brake control panel and its connections to the Main Brake Handle as Set Up mode is initiated. The Application and Release Valves are energised and the Main Relay Valve charges the Brake Pipe up to 3 Bar. The locomotive is being prepared for service and the brake control system is being set up.  The Brake Control Panel is provided with a connection to the Main Reservoir Pipe and is supplied with air from that pipe at 10 bar. There are also connections to the Brake Pipe and the Equalising Reservoir. On a 'dead' locomotive the Brake Pipe pressure will be at zero because the Emergency Valve (not shown) is de-energised whilst the locomotive is shut down.  When "Set Up" is initiated, the Emergency, Application and Release Valves are energised. The Release Valve is energised through the closed contacts "A" and those of the 3 Bar Pressure Switch.  The Application Valve is energised through closed contacts "A" and the <4.8 Bar contacts of the 4.8 Bar Pressure Switch. Energising the Release Valve causes compressed air, supplied at 5 Bar from the Pilot Reducing Valve, to pass to the Equalising Reservoir and to the upper chamber of the Main Relay Valve. The pressure in the upper chamber of the Main Relay Valve, being greater than that in the Brake Pipe, causes the port at the base of the valve to open and allows the Main Reservoir air supply to charge the Brake Pipe until it reaches a pressure of 3 bar.

The principal components of the Brake Control Panel are as follows:

A Pilot Reducing Valve which converts air from the Main Reservoir at 10 bar to a pressure of 5 bar - the control pressure required for the maximum pressure of 5 bar normally allowed in the Brake Pipe.

An Equalising Reservoir, not actually mounted on the Brake Control Panel, provides a volume reservoir by means of which Brake Pipe pressure is regulated. Variation of the pressure in the Equalising Reservoir will cause a corresponding variation of pressure in the Brake Pipe.

A Main Relay Valve which responds to changes in Equalising Reservoir pressure to control the supply of air to, and the exhaust of air from the Brake Pipe. If the Equalising Reservoir pressure in the control chamber at the top of the valve becomes greater than that in the Brake Pipe, the valve will open to allow the Main Reservoir supply to recharge the Brake Pipe. If the control chamber pressure is lower than that of the Brake Pipe, the valve will lift and open an exhaust to discharge Brake Pipe pressure. When the control chamber and Brake Pipe pressures are equal, both the supply and exhaust ports of the valve are closed and the Brake Pipe pressure will remain unchanged.

A Release Magnet Valve which, when energised, connects the 5 bar output from the Pilot Reducing Valve to the Equalising Reservoir and to the upper chamber of the Main Relay Valve.  When de-energised, the valve is lifted by a spring.

An Application Magnet Valve which, when energised, prevents a loss of Equalising Reservoir pressure. When de-energised with the Release Valve, it allows Equalising Reservoir air to escape and cause a reduction in Brake Pipe pressure.  When de-energised, the valve is lifted by a spring.

A 3 Bar Pressure Switch which ensures that a minimum pressure of 3 bar is maintained in the Equalising Reservoir (and thus the Brake Pipe). This provides an emergency pneumatic control pressure for the Brake Pipe. It should be noted that, under normal conditions, a pressure drop to a level of 3 bar in the Brake Pipe is sufficient to give maximum brake effort. A further reduction will not increase the brake effort.

A 4.8 Bar Pressure Switch which is used to overcome small leaks to maintain the Brake Pipe pressure at a fully charged level.

An  E.P. Brake Pressure Switch which detects brake pipe pressure and gives electrical signals along the e.p. control train wires to allow remote control of local control units on other vehicles with e.p. equipment.

Set-up Complete

Figure 2: When the brake pipe has reached the 3 bar pressure level, it can be said that the set-up is complete.  The control system looks like this: When the pressure in the Equalising Reservoir reaches 3 Bar, the 3 Bar Pressure Switch contacts open and cause the Release Valve to de-energise, closing off the supply from the Pilot Reducing Valve. The Application Valve remains energised. When the pressure in the Brake Pipe reaches 3 Bar, the Equalising Reservoir pressure detected in the Main Relay Valve equalises with that of the Brake Pipe and the port at the base of the valve closes under spring pressure. The air pressure in the Brake Pipe now remains at 3 Bar.  The Brake Pipe pressure is low enough to ensure that the brake remains applied.

Release

Figure 3: To release the Main brake, the Brake Pipe must be charged to a pressure greater than 4.8 Bar. This is done by selecting the Release position of the Main Brake handle. The handle must be held in this position against its spring pressure until the Equalising Reservoir gauge indicates 4.8 Bar. The Brake Pipe pressure will then rise to 5 bar under the control of the Pilot Reducing Valve.  For Release, the control switches and valves are reconfigured to the positions shown in the diagram. When the Main Brake Handle is moved to the Release position, contact "R" closes and causes the Release Valve to energise. The Application Valve will already be energised if the Brake Pipe pressure is below 4.8 bar (Contacts 'A' closed and <4.8 bar contacts closed). With both the Release and Application Valves energised, the Brake Pipe will begin to charge in the same way as occurs in Set Up mode.  When the Equalising Reservoir pressure reaches 4.8 Bar, the <4.8 Bar contacts will open and the >4.8 Bar contacts will close.  This will cause the Application Valve to de-energise but the Release Valve remains energised (through the >4.8 bar contacts) and open to allow the Brake Pipe to charge up to 5 bar under the control of the output pressure of the Pilot Reducing Valve.

Running

Figure 4: When release of the brakes is complete, the driver can allow the brake handle to return to the mid position and set the brake system to the "Running" position. The brakes remain released and the brake pipe pressure is maintained at the 4.8 bar pressure automatically as shown here, unless a rapid discharge of the pipe occurs or the Application position is selected. During normal running with the brake released and the Main Brake Handle in the Mid position, the Release Valve is held open by the contacts 'A' and the >4.8 contacts of the 4.8 Bar Pressure Switch. The energised Release Valve allows an upper level of 5 Bar to be maintained in the Brake Pipe under the control of the output pressure of the Pilot Reducing Valve. The purpose of this setting is to allow the brake control to automatically maintain brake pipe pressure against any small leaks or losses in the brake pipe. This prevents the brakes form "leaking on" and stopping the train unnecessarily. 

Application

Figure 5: In order to apply the train brakes, it is necessary to reduce the pressure in the Brake Pipe to below 4.8 bar. This will occur when the Main Brake Handle is moved to the Application position (against spring pressure) causing contacts 'A' to open and de-energise both Application and Release Valves, regardless of their former state. With the Application and Release Valves de-energised, air escapes through the Application Valve exhaust from the Equalising Reservoir and the upper chamber of the Main Relay Valve so that the pressure in the Brake Pipe becomes greater than that in the upper chamber and opens the valve to exhaust the Brake Pipe. The reduction in the Brake Pipe pressure causes the brakes on the train to apply. Because the Release Valve is de-energised, the connection between the Pilot Reducing Valve and the Equalising Reservoir is closed to prevent the loss of air from the Equalising Reservoir being replaced.

Maintaining a Constant Brake

Figure 6: Once the desired level of braking is reached, the driver should allow the brake handle to return to the Mid position.  This will energise the Application Valve through closed contacts 'A' and the <4.8 bar contacts of the 4.8 bar pressure switch. The Release Valve remains de-energised. With the Application Valve energised, the exhausting of the upper chamber of the Main Relay Valve ceases and the exhaust port of the valve will close when the Brake Pipe pressure equalises with that in the upper chamber. The Brake Pipe pressure will now be held at a constant level, as will the brake effort on the train.

Partial or Full Release

If the driver requires a full release of the brake, he must move the Main Brake Handle from the Mid position to the Release position.  The brake control panel will then appear as described above under Release and the Brake Pipe pressure will restore to 5 bar.

If the driver requires a partial release of the brake to reduce the retardation effort but still maintain an application, he must move the Main Brake Handle to the Release position until the required reduction in brake effort is achieved, then restore the handle to the Mid position. The Mid position must be selected before the Brake Pipe pressure reaches 4.8 bar otherwise the train brakes will be fully released.

When the Main Brake Handle is returned to the Mid position, the Release Valve will be de-energised by the opening of the contacts 'R'. The Application Valve will remain energised. Recharging of the Equalising Reservoir will cease and the supply of Main Reservoir air to the Brake Pipe will stop as soon as the pressure in the upper chamber of the Main Relay Valve equalises with that in the Brake Pipe.

Electro-Pneumatic Assistance

In addition to providing the driver with a means of controlling the pressure in the Brake Pipe from the locomotive, the Brake Control Panel is equipped with a pressure switch (the EP Brake Pressure Switch) which provides electro-pneumatic control of the brakes on each vehicle equipped with electro-pneumatic brake valves. The pressure switch detects differences in pressure between the Equalising Reservoir and the Brake Pipe and sends corresponding electrical signals to either an Application or Release Wire running the length of the train. The brake control equipment on each vehicle responds to these signals to apply or release the brake.

When the application position is selected, the Equalising Reservoir pressure falls below that of the Brake Pipe. The difference in pressure unbalances the EP Brake Pressure Switch and causes a contact to close and energise the application train wire. This results in a local brake application on each vehicle on the train.

When the pressure in the Equalising Reservoir and the Brake Pipe is equal, as during a constant level of brake application (see above), the EP Brake Pressure Switch achieves a state of equilibrium and the feed to the Application wire is broken. This holds the existing level of brake on individual vehicles.

When the Release position is selected (see above), the pressure in the Equalising Reservoir rises above that in the brake pipe causing the EP Brake Pressure Switch to unbalance and close a contact in the Release train wire to release the brakes on individual vehicles.