2004

Poor power quality makes cooker switch itself on

Typically, regular voltage quality spot checks are made throughout a local distribution system with additional measurements taken when a customer asks; is there a voltage problem? Indeed, this was the initial question that prompted an investigation into the cause of a modern electric cooker switching itself on.

Initial discussions between the local Distribution Network Operator (DNO) and customer revealed that this phenomenon first took place at Easter, then July and was experienced repeatedly throughout the summer months. However, the problem did not then re-occur until late September.

Standard voltage quality analysis - to BS EN 50160 'Characteristics of voltage supply in public electric power supply networks' - simply showed an increased level of flicker, only slightly above normal levels. However, investigations did reveal that a cable-car was located some distance away from the residential area. This cable-car comprised a double lift installation, having a 65kW lift with slipring motor compensated with 20kVAr capacitor power, as well as a 75kW lift with a B6-circuit rectifier drive. Although electrically distant from the local distribution substation, the cable-car was supplied from the substation by a two-core 2x95mm² copper cable, approximately 400m long.

Further investigations indicated that the oven malfunction seemed to coincide with periods of increased cable car use, normally at peak periods in the summer. A half-day network analysis at the lift equipment connection point in the local distribution system was carried out. Oscillograms were captured when the envelope trigger or transient level trigger varied by 20% of the voltage fundamental peak, or at least 65V between the cable and earth. Results showed that these limits were exceeded by a factor of two, by commutation spikes caused by the operation of the B-6 rectifier drive of the 75kW lift when it was the only lift in use.

Going back to the cooker manufacturer revealed that the cooker's electronic oven controls initiated switching commands via pulses. It was possible that the commutation spikes - with their steep slopes and zero-crossings - were being mistaken as switch-on commands.

Interference with gas cooker control creates safety risks

(Conformity, March 2006)

In one particular Canadian community, many residents had complained over a number of years about interference with gas cooking ranges and other appliances located in their homes. In March 2004, for instance, one resident found that her range was intermittently being programmed, followed by the oven operating at random temperatures. It usually remained in this mode until manually turned off.

Similarly, in March 2005, another resident found that she had no control over the functions of her gas range, other than the ability to turn the range on or off. A third complaint was registered in October 2005, involving a range of the same model, type and manufacturer. This particular range had been operating reliably for the prior four years in another a residential community west of Toronto. By coincidence, the homeowner in this instance was located next door to the resident involved in the second incident noted above.

The residents had no prior knowledge of what might have caused the effects that they experienced (the developer who built the community had told them that they were located too close to the airport!). Imagine their surprise upon learning that the actual source of their problems was that AM transmitter located half a kilometer away. Their surprise was even greater when they found out that the transmitter causing the interference was actually legally operating within the terms of its license.

Fortunately, all three of these potential fire hazards were resolved satisfactorily. But how many more ranges of the same type and model are out there, waiting for the "right" interference signal to cause a malfunction, perhaps even a disastrous one?

AC sags interfere with appliances

(Planet Analog, 30th July 2007)

We recently had a short and modest line-voltage sag at home, no big deal. While the incandescent lights dimmed slightly and came back up, nearly everything else was, as logic designers so elegantly say, "indeterminate." The PC chattered internally and went wild; some of the compact fluorescents lamps (CFL) came back on, but some did not; the displays on the oven and microwave became random segments; and the home network crashed but eventually reinitialized itself.

It was the plain 900-MHz cordless phone, a relatively simple device, which really showed the most bipolar behavior. The base station indicator LED said the phone was cradled and charging even though the phone was absent; the phone itself would not connect and provide dial tone. The solution was the usual tactic: unplug and restart. I unplugged the base unit from its wall wart; that cleared both the base station LED and its operating cycle. I still had to open the phone to disconnect/reconnect its battery, to reclaim that dial tone, though.

I know "stuff happens", but this particular stuff indicates both how dependent we are on decent line power, and how poorly designed many lower-cost products are for even slight dips in the mains. There are plenty of good voltage-monitor ICs available for line AC as well as low-voltage DC, but they cost money and board space. So end-users, your average customers, have to be prepared to go around and manually reset lots of their appliances.

To add to the frustration, many of these devices have soft, not hard, power on/off switch functions (see "When an on/off switch really does that"), so the user has to unplug the AC cord. That's not a big deal, unless you have to start moving some big furniture to get to the outlet. Would a whole-house line conditioner be a better idea?

Garage Doors

'First Responder' frequency tests interfere with garage door openers

January 2007

The Associated Press reports that recent testing by the U.S. Air Force of radio frequencies intended for eventual use by first responders has had the unintended effect of disabling automatic garage door openers in an area near Colorado Springs, CO.

The frequency testing in late November 2006 reportedly took place at the Cheyenne Mountain Air Station, the location of the North American Aerospace Defence Command. The effects from the transmission should technically have been limited to an area 15 kilometers from the test site, but Air Force officials suspect that the affected range was extended due to the height of the testing site (nearly 2000 meters).

As a result, hundreds of residents in the area surrounding the Station found their automatic garage door openers suddenly inoperable. One area company reportedly received more than 400 phone calls for assistance in fixing the disabled garage door openers. Air Force officials are said to be investigating how best to resolve the interference problem, and have discontinued the frequency testing for now.

New Pentagon system suspected of interfering with garage door openers

November 2005

A widespread problem with a mysterious radio signal that caused some garage doors in the Ottawa region to stop working has vanished. The powerful radio signal causing the problem stopped transmitting on Thursday afternoon, around the time CBC News contacted the U.S. Embassy to ask if it knew anything about it. The embassy denies that it had anything to do with it.

The signal was being transmitted at 390 megahertz, a frequency used by the Pentagon's new Land Mobile Radio System. The same frequency is used by garage doors openers, which started to malfunction around the city about two weeks ago. A similar problem has popped up around military bases in the States.

The world's biggest garage door manufacturer, the Chamberlain group, took the problem seriously enough to fly design engineer Rob Keller to Ottawa from its Chicago headquarters, with machinery to try to track the signal. But by the time he got there, the signal was gone.

Aircraft carrier interferes with garage doors in Hobart, Tasmania

(New Scientist, 24th April 1999)

Hobart in Tasmania suffered an unusual blight earlier this month. Residents all over town found themselves trapped in their garages when the remote controls that operate the garage doors suddenly failed to function. Roll-a-door companies were flooded with calls from angry garage owners and were at first completely nonplussed by the problem.....

Then the explanation emerged: the aircraft carrier USS Carl Vinson had just cruised majestically into the town's docks, equipped with navigational radar employing the same frequency as the remote controls for the town's garage doors. According to the local newspaper The Advocate, an apologetic Lieutenant Dave Waterman, the ship's public affairs officer, said that the problem would only occur when the ship was arriving and leaving.

(NB The USS Carl Vinson has gained a bad reputation elsewhere)