PDA

View Full Version : Another troubleshooting challenge


steveparrott
07-19-2010, 02:01 PM
A 600W transformer (with 80W load) shows 107V at the timer outlet but there's no voltage on the secondary taps.

Conditions and hints:

- A single wire run (with 80W load) is connected to the 12V tap and common #1.
- There's nothing wrong with the transformer (a second transformer was checked with the same results).
- The transformer contains no electronic components (that require higher voltage to function).
- The GFCI outlet checks out OK (also polarity is OK).

Why is there is no voltage at the secondary? What tests would you do to find out?

emby
07-20-2010, 12:16 AM
A 600W transformer (with 80W load) shows 107V at the timer outlet but there's no voltage on the secondary taps.

Conditions and hints:

- A single wire run (with 80W load) is connected to the 12V tap and common #1.
- There's nothing wrong with the transformer (a second transformer was checked with the same results).
- The transformer contains no electronic components (that require higher voltage to function).
- The GFCI outlet checks out OK (also polarity is OK).

Why is there is no voltage at the secondary? What tests would you do to find out?
Are the secondary transformer fuses or breakers tripped? Disconnect the secondary load and test taps.
Eliminate the timer and test. This takes the timer out of the equation. If there is nothing wrong then the timer is faulty.
If you still have no voltage on the secondary without the timer and there is nothing wrong with the transformer and its cord start by testing the voltage at the GFCI. As an electrician I can proceed to check the GFCI to ensure that its operating properly. If not change. If it is working properly than most likely there is a loose neautral wire not allowing the circuit to complete under load.
Plug the transformer into another power source and check the output for non electricians.
Thats my try.

The Lighting Geek
07-20-2010, 01:54 AM
Yeah, what Emby said...LOL

Pro-Scapes
07-20-2010, 10:10 AM
What is the threshold for the CAST relays to trip inside the transformer ? I know if you try to dim them they will chatter then not engage at all. I thought the useable minimum was much lower tho.

Possibly a really poor connection in the line voltage that falls flat on its face as soon as any type of load is put on it ? If you are 100% sure the trans is working fine then its obviously line voltage related. What else is on the circuit. Add a load tot he outlet such as a high powered work light or power tool and watch to see what your voltage does. Probably some corrosion in the GFCI from bad weather proofing.

steveparrott
07-20-2010, 10:48 AM
Are the secondary transformer fuses or breakers tripped?
The secondary magnetic breakers were not tripped.

Disconnect the secondary load and test taps.
You're getting close. When the secondary load was disconnected, the voltage taps measured about half the expected voltage.

Eliminate the timer and test. This takes the timer out of the equation. If there is nothing wrong then the timer is faulty.
Timer and photocell were removed and bypassed - no problem there.

If you still have no voltage on the secondary without the timer and there is nothing wrong with the transformer and its cord start by testing the voltage at the GFCI. As an electrician I can proceed to check the GFCI to ensure that its operating properly. If not change. If it is working properly than most likely there is a loose neautral wire not allowing the circuit to complete under load.
Ok, this is close enough. Something was wrong with the GFCI circuit - improper wire gauge, loose terminal screw connection - we're not sure. The low voltage at the outlet was the biggest tip-off. Other receptacles on the property measured about 120V. This one measured 107V.

This is a good reminder that when we find no voltage or lower-than-expected voltage on the secondary, then the GFCI outlet's voltage should be measured both 'with load' and 'without load'. I don't know what difference is acceptable, but keep in mind that even a low starting voltage (no load) should ring a warning bell. There should be less than 5% voltage drop between the feeder panel and the GFCI. (Read this article on the subject (http://www.psihq.com/iread/faqvolt.htm).) Voltage drops greater than 5% should be reported to the homeowner and an electrician should be summoned. The lost energy is converted to heat and could be a fire hazard - many house fires start due to this sort of faulty wiring.

Keep in mind that this input voltage-drop-under-load is not only a fire hazard, it is also a huge energy waster. A loss of 10% voltage at the input is equivalent to a 10% increase in energy usage. A responsible installer should insist that GFCI's are wired not just to meet minimum code, but rather are wired for minimum voltage loss under load.

Tim R.
07-20-2010, 11:32 PM
Ha, funny. I just encountered this last week, but found I had a bad transformer which was hard to believe since they bench test them before they go out. My voltage at the timer plug was 122.4 with no juice at my secondaries while unloaded. Replaced with another unit and good to go. It had to have been a loose neutral inside the transformer housing itself.

INTEGRA Bespoke Lighting
07-21-2010, 09:31 AM
And it should be noted that unless you are a licensed electrician, you should not be messing about with installing or changing or re-wiring 120V devices. Even things as simple as a GFI receptacle. Your livelihood as a Low Voltage Lighting Contractor is riding in the balance.

steveparrott
07-21-2010, 09:55 AM
And it should be noted that unless you are a licensed electrician, you should not be messing about with installing or changing or re-wiring 120V devices. Even things as simple as a GFI receptacle. Your livelihood as a Low Voltage Lighting Contractor is riding in the balance.

Absolutely! An excellent practice is to establish a good relationship with an electrician who understands the operation of low voltage systems. Most electricians have limited experience with issues such as inrush and NEC code as it applies to low voltage wiring.