Safety Contactor

[dave992]

Afternoon all,
I need some clarification regarding the use of a safety contractor and whether we can classify the safety contractor as a point of isolation. We have a control panel that requires inserting a castell key into the HMI unit to activate it. When we remove the castell key from the trap position, it disconnects the safety relay. Can this be used as the isolation point?
Thanks.

 

[James]

I would say yes and no.

it is most likely suitable for use as an point of isolation for mechanical maintenance.
however depending on how it is wired and what the safety systems in place on the machine are, it may not be.

this is one of the situations where people will have different opinions and the only correct one is a well thought out and documented Standard Operating Procedure.
followed up with detailed maintenance requirements if it is not going to be a simple "lock off main isolator before doing maintenance"

there is so much info needed to make a proper assessment of this that nobody can tell you remotely.
have you consulted the machine manufacturer?

 

[mainline]

Afternoon all,
I need some clarification regarding the use of a safety contractor and whether we can classify the safety contractor as a point of isolation. We have a control panel that requires inserting a castell key into the HMI unit to activate it. When we remove the castell key from the trap position, it disconnects the safety relay. Can this be used as the isolation point?
Thanks.

Contactors can be used for isolation, but not for safe isolation.

 

[davesparks]

Contactors can be used for isolation, but not for safe isolation.

Why not?

 

[James]

Why not?

Can't Lock them off?

 

[loz2754]

Table 537.4 indicates that contactors to BS EN 60947-4-1 are suitable for isolation as long as they are marked with symbol for isolation. With the caveat in 537.2.4 that devices for isolation shall be selected and or installed so as to prevent unwanted or unintentional closure. It then suggests means of achieving this eg padlocking.

I can find no differentiation in BS7671 between isolation and safe isolation. It is the same thing. At least according to the definition of isolation on page 37 "function intended to make dead for reasons of safety..."

 

[davesparks]

Can't Lock them off?

That depends on how the contactor is controlled,

 

[mainline]

Table 537.4 indicates that contactors to BS EN 60947-4-1 are suitable for isolation as long as they are marked with symbol for isolation. With the caveat in 537.2.4 that devices for isolation shall be selected and or installed so as to prevent unwanted or unintentional closure. It then suggests means of achieving this eg padlocking.

I can find no differentiation in BS7671 between isolation and safe isolation. It is the same thing. At least according to the definition of isolation on page 37 "function intended to make dead for reasons of safety..."

The only safe way of isolating the power is to open a switch designed for that purpose and ensure the switch cannot be closed accidentally. Contactors will have a higher rate of failing to open than a dedicated switch. I suppose one that locks in the open position may comply but I’ve never seen one. It will also have 2 supplies to contend with.

 

[mainline]

Table 537.4 indicates that contactors to BS EN 60947-4-1 are suitable for isolation as long as they are marked with symbol for isolation. With the caveat in 537.2.4 that devices for isolation shall be selected and or installed so as to prevent unwanted or unintentional closure. It then suggests means of achieving this eg padlocking.

I can find no differentiation in BS7671 between isolation and safe isolation. It is the same thing. At least according to the definition of isolation on page 37 "function intended to make dead for reasons of safety..."

isolation for reasons of safety could be a contactor that cuts the supply to a machine when a stop button is pressed it couldn’t be said to be safely isolated.

Safe isolation is more for maintenance and has its own procedure.


Existing safety standards give recommendations on how to apply contactors in functional safety applications. Regular operation of the equipment (opening and closing of the contactor) combined with a diagnostic control system (using the contactor’s mirror contacts) and a fault reaction function (system lock out, alarm, maintenance etc) enable the early identification of dangerous faults, increasing the safety reliability of the equipment. EV charger design standards already consider functional safety requirements in the basic design and consequently the design feature - electrical disconnection in the event of an open-circuit neutral fault built into the EV chargepoint, can be verified by reference to the existing standard 61851-1.

Contactors are designed to operate regularly (open & close) providing a simple and reliable method of remotely switching electrical loads on a regular basis. When using them in safety related applications, we should take note of the advice given in the contactor product standard 60947-4 and existing safety standards.