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How a Trailsnap Plug can prevent an inconvenience


Among caravan folks are a small group of gadget junkies who buy almost anything that is novel and possibly useful. Then there are a somewhat large group of caravan owners who take pride in their parsimony and get some kind of perverse satisfaction from doing everything cheaply.

There are a majority of us who look for a better way of doing something that needs improvement and will buy it if its offers a good value for money. Value for money is a very subjective assessment that will vary greatly with the individual’s needs and means, but ultimately it’s the factor that determines whether people will buy this upgrade or not.

Most people who have to deal with trailer plugs have found countless reasons to hate them. The metal ones, thought by many to made from aluminium, but actually cast from ZAMAC, which is an acronym for Zinc, Aluminium, Magnesium and Copper, corrode when they shouldn’t, and break whenever they come under stress mostly due to the seven pairs of brass contacts also corroding to the point that prevents them from mating.

To prevent corrosion, the contacts need to be kept well oiled, but the 7-pin plug, (being the part that attaches to the tow bar with three perennially annoying screws), hangs in the weather and is drenched by every splash the car encounters. The plug-top, (the part that goes on the end of the cable), is equally exposed to corrosion and additionally always seems to be hanging where it can most easily be damaged.

Inserting the plug-top goes something like this:

  • Inspect the contacts visually
  • Blow them hard to get rid of the dust and resident insects
  • Align the plug-top into the plug in way that seems to be right
  • Push really hard while wiggling the plug-top about (sometimes certain bowel gases may escape at this point)
  • Stand behind the trailer and yell instructions at an accomplice in the vehicle to turn lights on and off.

If something doesn’t make contact, take everything apart until you find the problem, and repeat the above.

If you were hoping for a better way to connect trailer lamps, please read on:

The TrainSnap Plug

The plug is a robust, completely weatherproof moulding with a 7-core cable entering through a tight-fitting grommet. It has a close-fitting, spring-loaded lid that will do a good job of keeping large grit out, but is in no way waterproof. By fully submerging the plug, water will pass by the lid and seep past the spring-loaded sliding contacts. Once water gets into the plug body, it will be nearly impossible to get out without drilling a drain hole.

Without immersion, the innards of the plug are well protected, which moves the corrosion problem to the end of the fitted cable – to where it is connected to the vehicle harness. If this is in an exposed place good protection practice will be required there. The plug has 7 cylindrical spring-loaded contacts of 3mm diameter which can protrude up to 3mm above the plug’s face. The current carrying capacity of a 3mm conductor is 63A, which is a lot more than the connector is rated for, so a wide safety margin assuming the connection with the mating plug top is efficient.

Many switches and most mating plugs have a quality known as ‘wipe’. This is where the two mating surfaces slide across each other when contact is made and, in doing so, scrape each other clean. The Trailsnap connection system does not have inherent wipe, meaning the two mating contact surfaces do not normally abrade against each other, but meet each other head-on without any lateral sliding movement. This increases the possibility of dirt on one or both of the contacts that will prevent a proper electrical connection.

However there is sufficient horizontal play allowed by the locating pin that contact wipe can be effected simply by twisting the plug top inside the plug once the two parts are pulled together under magnetic force. I would say you should definitely get into the habit of doing this little back and forth twist every time you make a connection because brass can and does corrode, leading to potential contact problems.

There are four powerful magnets positioned around the perimeter of the plug between the seven contact points, each of which is eagerly seeking out a partner to hold on to.

The Trailsnap Plug Mounting

The Trailsnap plug is secured by the same three screw holes used by the traditional version. Since these screws are also exposed to harsh conditions, galvanised hardware is sure to corrode quickly and stainless steel is the only long-term solution. The slotted holes in the plug body will accept M6 at a stretch but M5 is more sensible. The plug body overhangs the mounting footprint and obscures almost 50% of the hole’s diameter.

No doubt they wanted to maximise the plug body width to optimise magnetic retention force, but it means that you cannot turn the screws with a flat screwdriver without damaging the plug body above the mounting holes.

Since it is not outside the realm of possibility that these plugs will become a target for thieves, I’d suggest stainless steel M5 CSK Allen cap screws. Fewer thieves walk about with Allen keys in their pockets, and an Allen key will fit more easily into the socket of the cap screw and allow it to turn. When thieves do start carrying Allen keys it will be time to plug up the Allen key sockets in the screw heads.

Turning a stainless steel screw into a mild steel threaded hole often leads to thread galling, which would be very inconvenient here because it will jam the screws in the holes very tightly. If you have tapped M5 holes in your mounting plate, it would be better to drill the threads out and use Nyloc nuts behind the mounting plate, Nylocs being desirable since the plastic moulding can retreat and loosen a regular nut.

Since you don’t find these screws at your local Mica, you might have to ask the Trailsnap manufacturers to offer these as an optional extra. It would be nice to give a product this good the best mounting hardware available. Also needed to complete the installation are 14 red crimp ferrules to join the cables (staggered so as not to make the resulting joint too thick) and a length of heat-shrink tubing to cover the weld. Otherwise, there’s going to be a lot of twisted-wire connections covered with ghastly plastic tape coming apart at inopportune moments, and bring the entire implementation into question when it does. Making the right consumables available for ordering will benefit the makers in the long run.

The Trailsnap Plug Top

The Plug Top is a very robust moulding, which is totally sealed at both ends, with a tight-fitting grommet at the cable entry, and 7 flush-fitted fixed brass contacts distributed around the perimeter in mirror image of the plug.

Again, four powerful magnets are distributed around the perimeter between the contacts and positioned to mate with those in the plug. The brass contacts need to be clean, and the embedded magnets tend to pick up and stray magnetic material that comes within range. Fortunately, we wear pants, which are ideal for wiping plug tops clean.


The four pairs of permanent magnets that hold the two plug components are well-positioned and does a fine job. However, you should be aware of what could possibly go wrong here, so that you can prevent it from happening.

A magnetic chain is only as strong as its weakest link. Each link in the magnetic chain has a magnetic quality called permeability. The item in the chain with the lowest permeability effectively determines the strength of the chain.

  • The permeability of air and other non-magnetic materials is 1.
  • The permeability of common magnetic materials is at least 8000, but can reach 200,000 for the purest iron.

Below is a formula that determines magnetic force F:

The quantity µ is magnetic permeability. You can see that when the value of µ goes from 8000 to 1 the value of F will decrease greatly. Also you see below the line the value of r is squared, so when r (distance) increases the value of F will diminish at the square of that increase.

What this tells us is that when there is an air gap (or an air gap filled with grit) between the magnetic faces of these two components, the magnetic force between them will greatly decrease, and that decrease will be at the square of size of the gap.

So, keep those surfaces clean. Not clinically clean and germ-free – just without grit. All it takes is a wipe with a rag. By experimentation I found that Trailsnap will accept a fair amount of dirt before the magnetic force is seriously compromised, but you need to know where the enemy lurks, and this is the Achilles heel.

Value for Money?

I’ve taken to pricing everything in Checkers grocery packets, the same way Discovery channel measures the weight of an oil tanker in terms of London buses, and its waterline length in football pitches.

By that standard each of these plug components costs one bag of Checkers groceries, and that means they are excellent value for money. I can’t imagine ever going back to the old plugs, and I hope I never have to use one again.

When I first came across the promotion of these plugs I was a little put off by the hyperbole of the campaign – it made these plugs sound a bit of a snake-oil gimmick, whereas in fact they are a small but important step towards making life easier for everyone that hooks up a trailer. I can foresee Trailsnap becoming standard equipment on all but the cheapest of trailers.

To find out more information on the Trailsnap Plug including pricing, kindly email:

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