FAQ - Why does throwing my Hornby point short circuit trip my controller?

[Chrissaf]

This is genuinely a very frequently asked question. In recent months [relative to this FAQ posting date] the issue of Hornby points being related to ‘short circuit’ faults is an issue which is being raised by forum members on a near weekly basis. This FAQ describes the Hornby manufacturing fault and how to test a point to confirm the fault being present. This Hornby manufacturing fault affects all Hornby points and is not limited to any one particular point product R stock code. Assuming the issue at the factory is eventually resolved, then once stocks of faulty points have been flushed through the supply chain, then the need for this FAQ should reduce.

Description of the fault.

Most parts of a point are physically separated. But there is one location where it is possible for a short to occur. The two moving switch rails of a point both lead into the plastic frog area and reappear electrically as fixed metal rails on the other side of the frog. The input moving switch rails are electrically connected to their corresponding fixed output rails on the other side of the plastic frog. The paths of the two rails therefore have to cross over each other. This metallic path cross over is embedded in the plastic forming the frog. There is a common manufacturing fault where the insulation between these two cross over metallic paths has been compromised and the two moving switch rails are then in electrical contact with each other.

If on a PC, then right clicking the images below and choosing 'view image' will display a larger version.

Fig 1 below shows a normal point that does not have this fault.

The dotted box in the drawing represents the plastic frog where the metallic electrical pass-through for the two rails cross. Note that there is no electrical connection between the brown live rail and the electrically dead black rail. The electrical pass-through path [not indicated in the schematic dotted box] embedded in the plastic frog provides electrical continuity between the points fixed and moving rails as they pass through the plastic frog area.

Fig 2 below shows what happens to Fig 1 above when the manufacturing fault is present.

If the insulation between these two paths is compromised due to a manufacturing fault, the two rails that cross over are shorted together as now indicated within the dotted box. You can see that when the point is in the straight ahead position, the ‘Main Loop’ is not affected by this fault. This is because, as Fig 2 above shows, the moving switch rail leading to the siding is now at the same brown electrical potential as the straight through brown rail but it is physically isolated from any opposite polarity blue rail. Any locos that are parked on the siding will still remain stationary as the two rail voltage polarities are the same [in this case brown].

Notes:

1. If the point switched left route is not a dead end siding, but is instead another part of the main layout. Then subject to the layout design and power distribution arrangements, then this short circuit fault may trigger the controller regardless of the point position.
2. f the layout is DCC and Hornby R8232 DCC point clips are fitted, then the ‘short circuit’ will be permanent and trip the DCC controller regardless of the point position.
3. Note also that in this FAQ, a ‘Left Point’ is being used in the description. It should be noted that the same fault can also be present on a ‘Right Point’ as well as any other Hornby point products.

Fig 3 below shows what happens when this faulty point is switched to route a locomotive into the siding.

When you switch the point to route to the siding as shown in Fig 3, the part of the ‘Main Loop’ to the right of the point now has a short circuit as indicated by the red arrow current path. The short is seen by the controller and therefore shuts down.

Because the point is switched to the siding, the short circuit does not pass round the ‘Main Loop’ to the left of the point, because the moving [brown] switch rail at the top is not touching the fixed [brown] rail. However, as seen in Fig 1, if the point is part of a ‘Main Loop’ there is potential for the short circuit to propagate clockwise around the ‘Main Loop’ to short circuit the other side of the point, which also prevents any power reaching the locomotive motor.

Testing a point for this fault.

If this fault is suspected, then the most efficient way of testing for the fault is to use a ‘Multimeter’, preferably before the point is installed [or removed, if already installed].

Given the sheer number of points that seem to be leaving the factory with this fault, it is advised that all points should be tested first before installation.

With the meter placed on the measure 'Resistance' scale. Place the red and black meter probes on the three locations indicated in the image below:

If the point is good, then the meter should not read any resistance at all [infinity resistance] which is an 'open circuit' on all of those three measurement locations. Take each reading twice. Once with the point switched to the left, and again with the point switched to the right.

Subject to the meter type, infinity resistance will show on the LCD display as either the number 1 being displayed or a series of horizontal 'bars' -----

If there is a 'short circuit' then the meter LCD will display either a zero, or a number very close to zero or an equally low number. Any reading other than an 'open circuit' reading on any of the three test locations should be considered as a point being faulty.

Note that your fingers will conduct and potentially show a false reading on the Meter's 20M resistance scale, so make sure that you are not touching any metal parts at all whilst taking the measurements.

On the meter, the Black probe plugs into the terminal marked C for common. The Red probe plugs into the terminal marked Ω and the rotary switch should align with the 20M resistance scale setting or the highest resistance setting that the meter supports ... see image below:

You can pick up a cheap sub £8 Multimeter on eBay. Well worth such a small investment for some basic fault diagnostics capability to put you in a good position for the future.

Fault Resolution.

Since the fault is in a location embedded within the plastic below the frog area of the point, it is not feasible to perform a DIY repair. The point will need to be replaced with a new one.

Assuming the point is a new purchase, then the faulty point should be covered by Warranty, but note this. This fault is widespread and believed to be common to certain manufacturing batches. Thus your original supplier may end up sending you a replacement point with the same fault from the same manufacturing batch.

It has been reported on the forum that some members have had to arrange for a replacement of the replacement. This in itself might make some retailers question you as to whether the product actually has a fault or whether your own installation of the product is suspect.

It has also been reported on the forum that replacements sourced directly from Hornby have also arrived with the same fault still present.

If you are lucky enough to have purchased your faulty point ‘over the counter’ as opposed to ‘mail order’, it is suggested that you ask the retailer to open the product and test it in your presence with a meter [take your own if you have one] to prove it is fault free before accepting it. You could also then take the opportunity to show the retailer the ‘short circuit’ fault using the meter on the point you are returning.