Slightly OT: Lamp Test Pushbutton

Ken Roach

Lifetime Supporting Member + Moderator
Join Date
Apr 2002
Location
Seattle, WA
Posts
17,445
This is a little OT, but I would like some general input.

I have an machine control cabinet with an HMI as well as numerous standalone LED-bulb indicator lamps. Most of those indicators are driven by ordinary general purpose PLC output modules. One is driven directly by a VFD and another by a safety relay.

Because the application is unusual, we have an industry consultant who has crane industry (CMAA and ANSI standards are involved) experience.

He asked that I add a "Push To Test" button just for the indicator lamps, and furthermore wants it to be hardwired to the lamp indicators.

If I wire a push-to-test button daisy-chained to the positive terminal of each lamp, that will let any of the PLC outputs "back feed" to drive all of the lamps.

To avoid that, I could install a bank of small relays, so that I can individually apply +24V to each of the lamps in parallel to the PLC output. Or, I could install a bank of terminal blocks with diodes to block backfeeding.

But both of those seem wasteful of wiring and panel space.

My alternate proposal is that the lamp-test button only physically wires to itself and to a PLC input, and that the PLC reacts to a lamp-test request by turning on all of the lamps, then turning them each off one at a time.

That allows "push to test lamps" button to also mean "push to test DC power, PLC run mode, and control cabinet input and output fusing".

I could also treat it the way malfunction indicator lamps work on your car; when you start the car, they all light up, and the don't turn off until the associated self-diagnostic system completes its function. There are a couple of indicators for subsystems that can benefit from operator-initiated testing instead of functional demand.

Am I missing an obvious way to wire a lamp-test feature that doesn't involve a bank of relays or diodes ?

If you saw a "Push To Test" or "Push To Test Lamps" button on a control cabinet, would you expect it to solely test the indicator bulbs themselves ?
 
My alternate proposal is that the lamp-test button only physically wires to itself and to a PLC input, and that the PLC reacts to a lamp-test request by turning on all of the lamps, then turning them each off one at a time.

That allows "push to test lamps" button to also mean "push to test DC power, PLC run mode, and control cabinet input and output fusing".

I have seen it done several ways... but my vote goes for your "alternate proposal" in that it tests more of the system and not just the lamps.
 
This is a little OT, but I would like some general input.

I have an machine control cabinet with an HMI as well as numerous standalone LED-bulb indicator lamps. Most of those indicators are driven by ordinary general purpose PLC output modules. One is driven directly by a VFD and another by a safety relay.

Because the application is unusual, we have an industry consultant who has crane industry (CMAA and ANSI standards are involved) experience.

He asked that I add a "Push To Test" button just for the indicator lamps, and furthermore wants it to be hardwired to the lamp indicators.

If I wire a push-to-test button daisy-chained to the positive terminal of each lamp, that will let any of the PLC outputs "back feed" to drive all of the lamps.

To avoid that, I could install a bank of small relays, so that I can individually apply +24V to each of the lamps in parallel to the PLC output. Or, I could install a bank of terminal blocks with diodes to block backfeeding.

But both of those seem wasteful of wiring and panel space.

My alternate proposal is that the lamp-test button only physically wires to itself and to a PLC input, and that the PLC reacts to a lamp-test request by turning on all of the lamps, then turning them each off one at a time.

That allows "push to test lamps" button to also mean "push to test DC power, PLC run mode, and control cabinet input and output fusing".

I could also treat it the way malfunction indicator lamps work on your car; when you start the car, they all light up, and the don't turn off until the associated self-diagnostic system completes its function. There are a couple of indicators for subsystems that can benefit from operator-initiated testing instead of functional demand.

Am I missing an obvious way to wire a lamp-test feature that doesn't involve a bank of relays or diodes ?

If you saw a "Push To Test" or "Push To Test Lamps" button on a control cabinet, would you expect it to solely test the indicator bulbs themselves ?

Could you change out the pilot light to be both pilot light and pushbutton together? I have many in our cabinets, you press it and the light comes on, let go and the light goes off. And then you have a 2nd set of contacts so when your actual output or input is on, the light turns on also. This will save you from adding extra push to test buttons all over the panel.

Basically, each pilot light PB will have 2 sets of contacts (one N.O. for testing the light and one set of closed contacts for the actual PLC output).

Our operators do agree that pushing a "light test button" should only turn on the pilot light, nothing else.
 
I do not know what control system you are using but does the brand have something like AB 1756-OB16D with

  • Special Features: Field Side Diagnostics: No Load Detection, Output State Verification via Pulse Testing, Per Point E-Fusing
You could detect a lost load as soon as it occurred without having to push a button. Advantage is it does not only check the lights.



If something like this is not available I would put the button on the HMI and also display the state of each light on the screen. If a light is not in the same state shown on the HMI, you know there is a problem.
 
I don't it's possible to hardwire it without diodes or relays. For one thing, you're right, you can't just loop all of the pilot lights together or they effectively become one big pilot light. And for another thing, if it was possible, products like these and these probably wouldn't exist.

If I were in that situation, I'd be pushing for your PLC-based test function as well. Failing that, I'd use a specific purpose lamp test module like the ones in the links above (which yes is ultimately just a bank of diodes, but at least it's a bit more industrial and "designed for purpose" than just whacking a bunch of diodes across your terminal blocks).
 
Thank you, guys, for the input !

I had a feeling I was going to discover that I was re-inventing the wheel. I've never seen a "lamp test diode block" before, but that's clearly what they're for.

I used to see "push to test lamp" indicators back when I was working in old sawmills, and maybe that's where I picked up the distaste for large arrays of things the might be buttons and might be lamps.

I'll give the consultant the choice of old-school lamp testing, or functional test.

:site:
 
Do you realize there is a standard pilot light designed for this? It's called a "dual input" or "test input" type. Each module has 3 terminals; common, normal input and test input. The inputs each go through an on-board zener diode to isolate it from the rest of the circuits.
800T_DID_Schematic.jpg
 
I did not realize that such pilot lights existed; another new thing !

This system is a headache for parts selection because of the extremely high shock and vibration, so every new device seems to also bring a bunch of packaging and connection and reliability problems with it, so I wanted to keep the amount of wiring to a minimum.

Thanks !
 
Funny, I have used these diode blocks for lamp test. It must have been 25 years since I last saw one.

I dont see any argument for that the pushbutton must directly activate the lamps.
From a safety aspect, when the operator presses the button, he must see the lamps going on. That is fail-safe. It doesnt matter what the signal path is from the button to the lamps.

The dual input pilot light looks smart, but versus the diode block thing, I think the dual input pilot light is more complex since you need 2 wires going all the way from the control cabinet to the lamps.
 
Every system I have seen that was plc controlled and had a push to test lamp button, it was always the plc that drove the lamps via the push to test button being wired to a digital input.
 
We usually specify all our new equipment come with AB 800T PTT operators. They come with a contact block on the backside that will isolate any PLC circuit from the 'test' function when the pilot light is pressed.
 
I like the "functional test" option.
First, regardless of how the test is done, if there is a failure, the first part of the diagnosis is the same, check the bulb first then go from there.
Second, adding a hard wire w/diodes or a relay system adds failure points. The more hardware there is the more potential points of failure there is.
Last, as Ken alluded to, using the PLC to test the lamps not only tests the lamps but the lamp circuit and controls. It also adds a level of flexibility I.E. having the lamps come on when the system is started up (or whatever some creative mind comes up with).
 
You could also change out the lights to be lighted pushbuttons for "push to test".. Then you could do it at the "pushbutton" by breaking the PLC circuit when you push the light.
But you do not have 1 button for all the lights.
 
Last edited:
Back in the old days when we actually had hardwired control systems we used pilot lights designed with a "push-to-test" function built in (like jreaf described). I'm pretty sure we used A-B hardware like 800 series. They have dropped out of favor over the years with PLC controls. I agree that the function should simply turn on all the lights. The original purpose was to detect burnt out bulbs, and even though LEDs are not as failure prone, the function is the same.

Bottom line, if the statement stands that "it must be hardwired to the pilot lights". There is only one way to meet that requirement. You need pilot lights with a "push-to-test" feature.
 
These are standard in my line of work. Typically, everything has to be push-to-test rather than a pilot light lamp test pushbutton - so you hardwire the +24V to a N.O. contact on each individual pushbutton. If you use a single pushbutton to test the lamps, then the diode-input lights are required.

I agree your proposal makes more sense, but at certain plants and places, they require the hard-wiring, much like some places still requiring safety to be hard-wired.
 

Similar Topics

I have been tinkering with a little DIY remote access stuff lately, prompted by some recommendations and challenges on this forum. To my...
Replies
5
Views
1,819
I need a little sanity check and hopefully advice from some experienced users. I have a mechanism that we're driving with a JVL MAC3000...
Replies
4
Views
1,128
Looking for an economic way to monitor kWh and kVARh on multiple circuits (up to 10) in a single distribution board. The Square D Power Logic...
Replies
3
Views
1,701
Does anyone make a "positioning cylinder" like the Festo DDPC but with a digital encoder feedback ? I have an application where a 0.0005 mm...
Replies
5
Views
1,741
How do I calculate wire and breaker size for 2 motors starting simultaneously? Thanks
Replies
7
Views
2,348
Back
Top Bottom