Contactor / Relay Query

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Is a contactor or relay ok to be on all the time? im looking to get a fault led to come on when a breaker trips but it would mean the relay or contactor been on 100% of the time and i dont want the coils to burn out fast.

Thanks
 
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Is a contactor or relay ok to be on all the time? im looking to get a fault led to come on when a breaker trips but it would mean the relay or contactor been on 100% of the time and i dont want the coils to burn out fast.
There are countless relays used as 'power failure detectors' such as you describe. I have several here that have been virtually continuously energised for the past 25 years and all are still fine.

Kind Regards, John.
 
Do you have a particular application in mind. It would seem easier to approach it from the opposite direction ie have an led that is constantly lit when the power is on & goes off with the power, a plug in night light for example. That way you would have less power consumption.
 
Almost all relay coils are OK when permanently energised at or below the rated voltage but not all. Check the data sheet for the duty cycle factor versus voltage. If it is 100% at the rated voltage then it will be OK.

One problem that can arise when a relay is permanently energised is that it doesn't reliably release when de-energised. Some lower quality relays with a DC coil become permanently magnetised to a small degree over time and this together with an aged and weak spring means the armature remains in the energised position when coil is de-energised held by residual magnetic flux. A copper or plastic button on the armature or on the end of the core of the coil prevents this magnetic lock-up from happening as flux leaks at the gap in the magnetic path.
 
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One problem that can arise when a relay is permanently energised is that it doesn't reliably release when de-energised. Some lower quality relays with a DC coil become permanently magnetised to a small degree over time and this together with an aged and weak spring means the armature remains in the energised position when coil is de-energised held by residual magnetic flux. A copper or plastic button on the armature or on the end of the core of the coil prevents this magnetic lock-up from happening as flux leaks at the gap in the magnetic path.
Very true, and I've experienced that problem - but it is not an issue with AC energisation, such as the OP is proposing. I suppose it's theoretically possible that the relay could get 'mechanically stuck' after many years of energisation, due to a combination of a weak spring and possibly an accumulation of 'muck' - but I've never experienced that - have you?

Kind Regards, John.
 
Many breakers can have switches fitted that are operated by the breaker. Here is an example
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a combination of a weak spring and possibly an accumulation of 'muck' - but I've never experienced that - have you?
One occasion a machine was damaged when a motor over ran during an abnormal shut down. The only logical explanation was that a shut down contactor with 24 volt AC coil did not immediately mechanically release when the 24 volts was removed. The 24 volts was only present if the machine was running normally and removed when ever a fault was detected. From memory it was several seconds before the motor stopped.

Contactors for other motors fed from the same 24 volt did drop out. The fault could not be reproduced but for safety the type of contactor used was changed and changes made to the shut down circuitry.

Where safety is critical it is not unknown for there to be two contactors in series so that if one does stick closed the other will open the circuit.
 
Where safety is critical it is not unknown for there to be two contactors in series so that if one does stick closed the other will open the circuit.
Indeed - the redundancy of 'belt and braces' is always reassuring in safety-critical situations.

In another thread today, I've seen a similar suggestion in relation to RCDs in series (and the same could even be said about MCBs in series - particularly given that we have no way of testing MCBs) - and, again, if it's a sufficientyly safety-critical situation, one cannot knock such redundancy.

Kind Regards, John.
 
particularly given that we have no way of testing MCBs) -
I have tested them with a current from a heavy duty 12 volt transformer and some large low ohm resistors.

if it's a sufficientyly safety-critical situation, one cannot knock such redundancy.
I no longer have the article could become a network of 13 valves in order to ensure that the over all open / closed function was achievable even if 2 or 3 of the valves failled.

The basic was two in series ensured the flow could be stopped if a valve jammed open. Then two more in series and in parallel with the first chain of two to ensure the flow could be enabled if one valve jammed shut. It got complicated when a valve linked the centres of the series chains in case two valves jammed shut and this valve then had to be duplicated to be fail safe.
 
Where safety is critical it is not unknown for there to be two contactors in series so that if one does stick closed the other will open the circuit.

We used to fit contactors in series to prevent overheat on our factory presses. It used to be the norm to fit two contactors of different makes just in case there was a chance that one of the makes were susceptible to sticking contacts (as they would both be carrying the same current)
Also the control circuitry would be arranged so that only the controlling contactor would make and break current. The safety contactor would make and break with no-load on a reset system.
 
particularly given that we have no way of testing MCBs) -
I have tested them with a current from a heavy duty 12 volt transformer and some large low ohm resistors.
I should have said 'no practical way of routinely testing MCBs'. If we wanted to verify disconnection times (as we do with RCDs), we would need at least 160A for a B32 (or 250A for a B50), and even more ridiculous/dangerous currents for Type C or D MCBs. The dangers of such tests would probably outweigh the benefits!

It's often occurred to me that this problem could be largely overcome if OPDs incorporated a current transformer, such that the actual sensing was always done at fairly low current, regardless of the In and current being interupted. If that were the case, everything but the current transformer itself (unlikely to fail in any way other than 'completely') could be tested at relatively low current, if an approprtaite test terminal was provided.

I no longer have the article could become a network of 13 valves in order to ensure that the over all open / closed function was achievable even if 2 or 3 of the valves failled. The basic was two in series ensured the flow could be stopped if a valve jammed open. Then two more in series and in parallel with the first chain of two to ensure the flow could be enabled if one valve jammed shut. It got complicated when a valve linked the centres of the series chains in case two valves jammed shut and this valve then had to be duplicated to be fail safe.
Yes, I've seen such hyper-redundant systems. Indeed, in the dim and distant past, I actually attended a course on 'redundancy'. I would suspect that such extreme measures may be used in such places as nuclear power stations, and maybe also in some applications in the aerospace industry.

Kind Regards, John.
 
I seem to remember where a fault can cause irreversible damage to personnel then the safety system protecting personnel must be of the duel type with two contacts with every E/Stop and gate switches. Although it seemed one could use a single sensing relay being designed for the job. In some cases a N/O and N/C contact was used.

It caused me some problems when contactors were replaced with semi-conductor versions so could no longer be included in the safety system so extra safety contactors needed fitting. Also a problem when losing power could cause a device to drop or open last thing one wants when some one is injured is hitting the E/Stop resulting in concrete being also poured over them.

However there is nothing in the post about it being a safety system all it asked was if a contactor or relay can be powered for a long time. And with a few exceptions the answer is yes.

I have also had problems with DC where a pull in and hold in coil are used. Due to some dirt the contacts failed to open and it overheated and failed. This can also happen with under voltage. The old CA45 starter motor was known for the problem with under voltage causing the process to stall and the resistor burning out.

However I would think likely working with AC and I would not think these odd places where it would not work would concern him?

I would go for aux contacts however the only problem is where type tested distribution units (Consumer units) are used with no ability to space the MCB's at 1.5 units wide. However I am sure one could fit a half blank as under the 12.5mm limit there should be no problem even if it can be removed without a tool.
 

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