Hardwired

[thama]

What does hardwired really means? What is the proper definition for it? I'm responsible for control & monitoring system for a ship under engineering scope. In the contract with the client, it says that the emergency stop for chilled water plant should be hardwired. Does that mean emergency stop through software connected to the plant using wires is sufficient?

[P Flados]

I would say that a physical dedicated switch needs to be directly wired into the circuit to ensure the action will occur without relying on any electronic devices at all. Those of us that have dealt with real world digital based control devices have learned that some functions are just too important to take any chances with at all. We trust when an emegency stop switch breaks the circuit that keeps a pump contactor picked up. We do not trust stuff that relies on a digital based device that can have some hidden flaw or failure mechanism that can make it stop responding to inputs. Equally bad is where a problem can allow a piece of equiment to start up after someone has "secured it" and then started workign on it.

[thama]

the definition really helps...thanks a lot...

[CDK]

Hard wired is any electrical or electronic circuit that does not require software or firmware (= embedded software) to perform its functions.

[michael pierzga]

You might contact a marine electrical engineer for clarification of the term, but as CDK states Ive always considered that hardwired means a physical make or break switch on the control circuit. NOT A RELAY. Relays may weld shut and the system will run out of control. A hardwired installation insures that power to the control circuit can always be interrupted for service work or system malfunction..

[philSweet]

I don't have a problem with a relay or contactor or other discrete component in the circuit, as long as its uninfluenced by ones and zeros coming from elsewhere. Hardwire doesn't ride the bus to work.

Its a crappy contracting description though. They are basically telling you what they do not want instead of what they do want. There is no guessing what they might consider to be sufficient.

Suggest to them that you interpret it to mean an analog circuit of discrete components and see what the client says.

More important is the general requirement for failsafe and lockout/tagout as already mentioned.

Are you having an issue where the supplier is giving you a system that comes with a possibly noncompliant shutoff? Nowadays, startup and shutdown of anything more complicated than a kid's train set is probably governed by software. The spec sounds like it's from the pushbutton and relay days.

I might also ask the client if the stop button is wired to a terminal in the chiller controller that is specifically intended for that purpose, is that acceptable, even if the controller then uses software to shut the system down. As integrator, your responsibility should end where the supplier obviously has picked it up, as evidenced by them labeling a terminal for emstop.