Advice sought - mains breaker tripping - Page 1
Forum section: electronics / general

author: aroby
author_rank: Regular Contributor
timestamp: May 16, 2014, 02:03:54 pm
content: I have a circuit at home where an Intermatic ST01 wall timerhttp://www.intermatic.com/Products/Timers/InWallTimers/Electronic_Timers/ST01_Series.aspxswitches two outlets.Into each outlet is plugged a landscape lighting transformer. All of this is inside the house. With one of the transformers (A) plugged in, this all works fine. When the other transformer is plugged in (B), the circuit runs for a while and then all of a sudden, there is a tremendous buzzing that causes the conduit carrying the wires to vibrate, then the breaker trips.This is a 15A circuit, the transformers together don't pull anywhere near that in normal operation and both are only light loaded on the output. The transformers both run fine if they are plugged into non-switched outlets. I also confirmed that there is no wiring issue - I bypassed the ST01 timer switch and ran both outlets together with no problem. Interestingly, when I did that transformer A was a lot quieter (they always have a small buzzing).I am assuming that the problem is a combination of the type of timer switch and some form of inductive feedback from the transformers, but I'm not quite sure how to fix it. Would a relay be the best approach here? If so, any suggestions of one?Thanks for any helpAnthony

author: Stonent
author_rank: Super Contributor
timestamp: May 16, 2014, 03:29:49 pm
content: Get a Kill-A-Watt meter and put it between the load and the plug. If the wattage goes off the scale before the breakers trip then you've got an issue.You could also use a surge protector that has a built in breaker to see on what side of the outlet the problem exists.

author: nihilism
author_rank: Regular Contributor
timestamp: May 16, 2014, 03:44:56 pm
content: You're probably exceeding the load rating for the timer for that particular type of load. What are the ratings on the transformers and the model number of the timer?

author: London Lad
author_rank: Regular Contributor
timestamp: May 16, 2014, 03:49:56 pm
content: Get a timer that uses a relay to switch the output

author: aroby
author_rank: Regular Contributor
timestamp: May 16, 2014, 04:52:08 pm
content: Quote from: nihilism on May 16, 2014, 03:44:56 pmThe transformers use less than 5A each and the switch rating on the timer isResistive (heater): 15 Amp, 120 - 277 VACTungsten (incadescent): 15 Amp, 120 VAC; 6 Amp, 208-277 VACBallast (magnetic): 8 Amp, 120 VAC; 4 Amp, 208-277 VACThis is what I thought would have been the problem, but apparently not. I can hear the transformers buzzing and sometimes pulsing when they are connected to this switch. The buzzing is significantly less, although there is still some pulsing, when they are not connected to the switch.Anthony

author: aroby
author_rank: Regular Contributor
timestamp: May 16, 2014, 04:55:53 pm
content: Quote from: London Lad on May 16, 2014, 03:49:56 pmI am pretty sure that that is how this switch works. You can hear the relay switching. But there does appear to be some interaction between the timer and the transformers that causes the strange behavior.Anthony

author: aroby
author_rank: Regular Contributor
timestamp: May 16, 2014, 04:57:04 pm
content: Quote from: Stonent on May 16, 2014, 03:29:49 pmI'll try the surge protector and see what happens. The Kill-A-Watt idea is good, but I'd have to stand there for a long time watching it! The behavior is not predictable.Anthony

author: Fank1
author_rank: Regular Contributor
timestamp: May 16, 2014, 10:20:10 pm
content: You have an intermittent short in the light circuit somewhere.When conduits vibrate it's always a short.

author: aroby
author_rank: Regular Contributor
timestamp: May 17, 2014, 03:36:09 am
content: Quote from: Fank1 on May 16, 2014, 10:20:10 pmThanks for the tip. I'll pull the wires out tomorrow and check them.Anthony

author: nihilism
author_rank: Regular Contributor
timestamp: May 17, 2014, 03:38:21 am
content: I think the problem lies with the statement in the spec sheet for the timer which states "no neutral required", which would indicate to me that the timer itself is being powered in series with the load. This would require a voltage drop between the input and output of the timer. Also your load would be classed as magnetic and is probably more than the 8A allowed. I think you're getting a greater than normal volt drop across the timer causing lower voltage into your transformers than desirable and this issue.

author: David_AVD
author_rank: Super Contributor
timestamp: May 17, 2014, 04:10:00 am
content: If it is one of those inline units (no neutral), then it will use a TRIAC and not a relay. It will also be stealing a little bit of power for itself by phase control when the load is on. This can be a problem for non-resistive loads.

author: London Lad
author_rank: Regular Contributor
timestamp: May 17, 2014, 09:05:55 am
content: ^^^ What he said

author: aroby
author_rank: Regular Contributor
timestamp: May 19, 2014, 01:55:23 am
content: Quote from: David_AVD on May 17, 2014, 04:10:00 amIt has a relay. There's a battery in it that I believe powers the relay. You can hear it click on and off.Didn't get time this weekend to pull the wires out of the conduit ...Anthony

author: David_AVD
author_rank: Super Contributor
timestamp: May 19, 2014, 02:36:35 am
content: Interesting. I've never seen a control device with a relay that doesn't use the neutral.

author: nihilism
author_rank: Regular Contributor
timestamp: May 20, 2014, 03:05:21 pm
content: ...unless the battery powers the whole of the timer circuitry...

author: SeanB
author_rank: Super Contributor
timestamp: May 20, 2014, 06:19:35 pm
content: You can do it with a latching relay, using a pulse to set and reset it, with the battery being charged when it is off. This has been done with industrial true delay off relays, using a simple circuit using a 4V8 NiCd pack charged from the input, and used to power the delay circuit and having enough charge to reset the relay when needed.