Investigating street light interference: SLI

Investigating street light interference: SLI
Posted by: Gary M. Rowe
Category: Anomalies /

This page contains a practical guide to investigating apparent Street Light Interference (SLI) . As well as providing instructions for researching SLI, this article also examines some of the possible causes. It will therefore also be of interest to those who have experienced SLI or are simply curious about the subject.

(Please note the safety advice contained in this article - never touch a street lamp. If it is subject to SLI, it may be faulty and dangerous!).

SLI is an ability claimed by certain people to switch street lamps on or off merely by being in the vicinity. We do not know who was the first person to make such claims, or indeed how old the phenomenon might be. I have only been involved for little over twenty-five years, but I suspect that it is as old as street lighting itself.

SLI is closely allied to TLI or Traffic Light Interference, which is far more commonly encountered. In traffic light interference the driver of a vehicle believes that by simply ‘willing’ a change to occur, he is able to influence the traffic lights in his favour as he approaches.


The investigation of SLI is divided into four equally important areas of research:

1) The investigation of anomalous SLI, interactive with a person, but whose cause is the result of an electrical or mechanical fault.

2) The investigation of anomalous SLI, interactive with a person, but whose cause is not the result of an electrical or mechanical fault.

3) The investigation of anomalous SLI which is not caused by any fault in the equipment, or by any mental or physical intervention by a person who is only coincidentally in proximity.

4) The investigation of anomalous SLI, interactive with a person, caused by that person’s paranormal abilities.


A) SLI can only be investigated to a satisfactory conclusion on a case by case basis.

B) NO scientific study could be conducted or indeed should be attempted without sufficient data.

C) As electrical apparatus of a technical nature is involved in SLI, only technically qualified/able investigators with legal access to, and a knowledge of, the system under study would be capable of eliminating the ‘normal’ explanations.

D) Would-be investigators without technical knowledge of street lighting should be dissuaded from attempting any personal investigations of the lamp itself. Lamps that display any features out of the norm and, in particular, turn on or go out at approach, are odds-on to be faulty and should be considered highly dangerous. Under certain fault conditions simply touching the metal lamppost could prove fatal. Apart from the lethal 240 volt electricity supply, many types of street lighting employ voltages at least double that of the mains. Explosions can occur in faulty components if vibrated. On several investigations I have found the ground around the lamppost to be at mains potential.

E) The following questions, if answered, will generate the kind of data any genuine scientific investigator would need as a minimum before any valuable assessment could be carried out:

i) How often have you observed the effect?

ii) What was the exact date and time of each observation?

iii) Was the observed effect a lamp or lamps coming on? Going off? Some other effect?

iv) Did the observed effect occur to just one particular lamp? A string of lamps? Or lamps in several different locations?

v) What was the exact location of the lamp or lamps?

vi) What type of lamp was involved?

* Sodium (bright yellow).

* Filament (normal light).

* Mercury vapour (bright blue/green white light).

* Fluorescent (diffused white light).

* Some other type?

* Not sure / do not know.

vii) How was the lamp mounted: On a steel lamppost? On a wooden post? Wall mounted? Suspended by wire? Other?

viii) How did you approach the lamp? On foot? In a vehicle? On a bicycle?

F) Only after a thorough technical investigation has failed to find a ‘normal’ explanation for the anomaly should further questions, such as those outlined in the SLI Effect, be asked of the witness to prevent the generation of a whole mythopoeic field.

G) Any person who has the psychical/paranormal ability to affect mains-operated, heavy wattage lamp equipment should be equally able to affect very low voltage/wattage equipment. This suggests that a common battery-operated torch would be the ideal apparatus on which to test such abilities.

H) If a case is discovered where tests have revealed that a mechanical switch or connection was manipulated without physical intervention then psychokinesis tests can be carried out on the subject.

I) Today many houses, vehicles, boats and pedestrians are equipped with radio transmission equipment. For example: cell phones, citizen band equipment, amateur radio transmitters, police personal radios and taxi cab radios.

Much of this equipment is of considerable output power and many times I have investigated cases where cooking stoves, electronic organs and even denture fillings have been the receiver of these transmissions. Lampposts and overhead cable lines can make very effective aerials. Many types of public lighting use inductive components as a part of their operating apparatus. This suggests that it might be worthwhile conducting some tests to determine if radio transmitters could be responsible for any random operations of lighting equipment.


There can be many explanations for lamps going on or off as they are approached. Here are a few:


Few pedestrians are ever more than a short distance away from a street lamp. Statistically, given the number of people and the number of street lights in the UK, there would have to be at least one person who is at this very moment approaching a street light that is either about to come on or go off.

At certain times of the day, namely around dawn or dusk, the mathematical probability of a chance observation of a lamp operating on approach (on or off) increases astronomically.

Street lights are turned on or off at least once in each twenty-four hour period, so there is unlikely to be anyone who has not witnessed such an event in his or her lifetime. We tend to take street lighting for granted, so it is not surprising that few of us will remember having seen this event.

Though it is planned that most of Britain’s street lighting should be turned on at around dusk and off again at around dawn, there are many reasons why street lights do not always follow this pattern. In the section headed ‘FAULTS’, I list a few of the common causes that can give rise to a belief that street lamps are operating because of your approach.

Most street lighting repairs are carried out during the daylight hours and may necessitate turning on the individual or string of lamps for testing. An individual’s approach could easily coincide with just such an event.


Apart from the normal switching of public lamps at the desired times, mechanical and electrical faults can produce some quite amazing effects where the witness unwittingly interacts with the faulty equipment or its electrical supply.

I list here a few that I have personally encountered during the course of my investigations:

Connections: By far the most common are poor electrical connections. The approach by the subject causes vibrations through the ground that disturb the faulty connection, causing either a make or break to the circuit and the associated light comes on or more often goes out. Tall lampposts act as vibration amplifiers transmitting the amplified movement to the lamphead, where even the most delicate approach can modify a lamphead fault.

Intermittent faults in the lamp’s supply cable can be quite delicate vibration sensors converting foot pressure or vibration into electrical contact.

Photocells: Anything that obscures the photocell (usually mounted on the top of the lamphead) from receiving sufficient natural light will cause it to operate the circuit. Some of the causes I have come across include: birds nests, roosting birds, birds droppings, autumn leaves, overgrown trees, litter, lamppost paint, traffic grime and splashes, passing clouds, fog and mist, snow, the cell’s becoming dislodged and pointing downwards, shade cast be high-sided vehicles.

Naturally, the removal of anything obscuring a photocell can turn a lamp on if there is sufficient ambient light.

If a photocell has just determined that it must turn on the lamp due to insufficient ambient light, any increase, natural or unnatural, in the light falling onto the photocell will reverse the process.

Some examples of how this can happen are:

(a) the turning on of internal house lights when a lamp’s photocell is adjacent to a house,

(b) the operation of any powerful security lighting in the vicinity of the lamp’s photocell,

(c) a vehicle’s headlights,

(d) reflected light from any source.

Clocks: Apart from the natural time when a clock is set to operate a circuit (on or off), other factors can modify the switching moment and make it interactive with a person’s approach.

When a clock mechanism reaches the pre-set point at which it should operate, ie. close or open its contacts, it encounters increased mechanical resistance to its forward rotational motion as its energies are divided between moving the dial and lifting the contact arm. Sometimes this additional pressure is sufficient to actually stop a poorly maintained clock at this critical moment. In any event, all clocks at this point of operation are highly sensitive to vibration. Traffic passing or even very small amounts of vibration, such as the approach of a person, can cause a light to operate.

All electrical contacts eventually become dirty or even pitted, particularly if used to switch heavy loads, ie. more than one lamp. Contacts damaged in this way may not make good contact even when closed due to the high resistance of carbon deposits. Such a condition makes the lamp liable to be operated by vibrations.

Temperature: Changes in temperature can cause electrical faults able to put lamps on or turn them off. Internal lamppost wiring is usually done with copper wire. Apart from its good electrical conductivity and low resistance, copper also has a good rate of expansion and contraction and its heat conduction is well exploited in the catering industry. An expanding or contracting copper wire puts mechanical pressure on its connection terminals which, if not tight enough or corroded, become susceptible to vibration. For reasons stated previously, the lamphead connections are susceptible to vibration. Many photocells are mounted under a transparent plastic cover on a printed circuit board with other electronics. As this board is exposed to direct sunlight and the print on the board is copper, it is prone to circuit board cracks that can be susceptible to vibrational operation.

Filament Failure: It is easy to overlook the fact that in filament lighting a piece of extremely thin metal (the filament) is heated to a degree where it is liable to break. Only a very small amount of vibration would be required to cause a lit bulb to blow.

Stroboscopic Effects: The stroboscopic effect, at its greatest in mercury lighting, can give rise to many unusual optical effects. For example, if you are driving at speed under a row of mercury lights when you may get the feeling that each lamp only comes on just as you reach it. The effect is more pronounced if the head is turned so as to look obliquely towards the lamp. This is because the “side” of the eye is the most sensitive.

Power Failure: Any power cut or resumption due to any reason, at any distance, will turn lights on or off on the same circuit.

In Summary: Finally any of the following items related to street lamps can be the cause of ‘natural SLI’: transformers, chokes, capacitors, connectors, clocks, circuit boards, cable faults, power cuts, photocells, lightening, contraction, vandalism, dogs, corrosion, water ingress, bulbs/tubes/filaments, fuses, contacts, lampholders, vibration, temperature changes, wind, birds, expansion, stroboscopic effects, ground resistance.


The following is a list of equipment that I have found to be invaluable when investigating SLI cases.

Watch: The recording of precise time in SLI events is the single most important piece of scientific information that should be gathered.

Lighting-up Timetables: Most helpful to determine if SLI is associated with normal operation of lamps on a given day.

Photographic Light Meter: Used to measure light at the moment of switching on or off when lights are operated by photocell. Repeated measurements can help to solve many cases with ‘normal’ causes.

Tape Measure: In conjunction with a sketch made of the lamp and its immediate vicinity, a series of measurements can reveal if the subject or indeed investigator is able to cause the effect in only one particular spot. If this is found to be the case, it often indicates that the cause is a supply cable fault. If it is found that both the investigator and the subject can produce SLI at any approach angle, then there is a strong possibility that vibration is the prime cause in association with a loose connection. In both instances the appropriate street lighting authority should be notified.

Vibration Detector: This instrument can be easily constructed out of scrap materials. It is used to determine if the ground surrounding a particular lamp is prone or susceptible to vibration. The device works in a similar way to a lamppost inasmuch as it can be adjusted to amplify vibrations to the point where they are easily detected. As the weight is adjusted upwards the device becomes more sensitive to the vibration transmitted through the ground on which it is placed. Marks on the upright rod can be read off like a ruler and recorded to build up records that will help in other cases.

The length of the upright pendulum is determined by experimentation in conjunction with the associated weight. It is possible to change the range of the instrument by changing to a different weight.

NOTE: Unfortunately, we don’t have a picture or diagram of Gary’s vibration detector. You may need to use a commercially available accelerometer instead.

Dual Torch: A portable battery-operated torch of the type that incorporates both a filament lamp and a low power fluorescent lamp can be used to conduct tests on site to see if external excitation is responsible for a light being lit when it is not connected to the supply. In this role the torch is not turned on during tests. If a positive test indicates that external excitation is responsible, then take a careful note of any adjacent transmitters, overhead power lines that may be responsible. Where tests prove positively that external excitation is the culprit, the same torch can double as the test apparatus in experiments to see if your subject can influence the operation by paranormal SLI using either excitation or psychokinesis.


If you are not able to conduct the physical/electrical investigation yourself, then “knowing a man who can” is the next best thing.

You will find that if you report a possible fault to the man responsible for your local public lighting (this may be the electricity company, a private contractor or the local council) he will consider it to be a favour, and you may find that he would be willing for you to ring back in the future so that he can tell you of his expert findings.

Note: You should regard the account above as forming general guidelines, not as hard and fast instructions, for developing your own experimental protocol. You are strongly advised to consult widely with suitably knowledgeable people (particularly engineers) before starting any SLI research. Neither ASSAP, nor the author, are responsible for any similar research you may do.

Author :Gary M. Rowe

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