1734-OBS8 - I need to switch to ground.

RetiQlum2

Member
Join Date
Feb 2024
Location
Arkansas
Posts
64
I have a device that requires breaking the 24V return for safety, rather than putting power on the circuit.

Can I do this with the OB8S or do I need to use a relay?
 
Using an OB8S on a general relay to break the common, is generally not safe at that point. A simple short to ground on that return renders the OB8s and relay moot.
 
Using an OB8S on a general relay to break the common, is generally not safe at that point. A simple short to ground on that return renders the OB8s and relay moot.
Until you work on machinery built in Europe. Those guy love NPN sensors to sourcing inputs and sinking outputs. And I agree that if the wire gets grounded out it's bad because things then turn on that shouldn't be on, and the PLC thinks are off.

They say it's because NPN devices respond faster, but when I asked Why that mattered when the PLC was only scanning the input at a much slower rate and that difference would only make an input read high one scan earlier once out of every 1,000 times it goes high, they couldn't provide a valid answer. Plus they were just regular inputs, not FAST-DC or HSCE.
 
Until you work on machinery built in Europe. Those guy love NPN sensors to sourcing inputs and sinking outputs. And I agree that if the wire gets grounded out it's bad because things then turn on that shouldn't be on, and the PLC thinks are off.

They say it's because NPN devices respond faster, but when I asked Why that mattered when the PLC was only scanning the input at a much slower rate and that difference would only make an input read high one scan earlier once out of every 1,000 times it goes high, they couldn't provide a valid answer. Plus they were just regular inputs, not FAST-DC or HSCE.

Must be an Asian thing too. All the Yaskawa servo drives from the 90s and early 2000s were NPN.

In regards to this topic, even a "safety relay" isn't safe if it's simply being operated by a single channel OB8S, and a single contact, breaking the neutral to the device. The OB8S could deactivate the safety relay, and the neutral through the contact get shorted and the device operates. "Safety Relays" mean squat if the implementation is still poor.

Safety is a complex design, and most safe systems installed by inexperienced people are not safe at all. Or at least aren't achieving the safety rating they desire.
 

Similar Topics

Hello everyone, how are you? I need to connect my 5-wire encoder to this high-speed output card. My current connection is on the first card in...
Replies
2
Views
74
Hi Guys, I tried to set the ip address of the 1734-AENTR series b using the BootP -DHCP Tool with no success. Is it possible to set the point IO...
Replies
7
Views
150
Good evening, I'm troubleshooting a 1734-AENT that I'm hoping to get some fresh ideas about. This is on an existing installation that has been...
Replies
10
Views
208
Hello guys, quick question here about RTDs. I have the TSSENOR00007 3 cable RTD(black, black, red) the only input module I have is the 1734...
Replies
2
Views
115
Hi I'm using AB PLC (5069-L350ERS2) with 1734 Safety IO Module. I add the module into my project the data tag is auto set to safety class. I'm...
Replies
13
Views
300
Back
Top Bottom