Wiring Through a Relay

If the load on each of the three circuits is equal, there is no logical reason for wiring the system using the contacts in series.

It can be argued that wiring in series increases the current switching capability - but I've also seen pretty convincing arguments to the contrary.

John, I don`t see that wiring in series increases the current capability as the current flowing through both relays is just the same.

It migt be security aspects to use two (or even more) relays in series as the possibility of "hanging" contacts decreases with the number of relays (the failure rates are multiplied in this case giving a remarkable longer Mean Time Between Failure, MTBF). Another question is whether both relays are primarily wired in parallel or if they can be closed by different sources - this might be another security aspect or operators requirement.

Regards Uwe

"I don`t see that wiring in series increases the current capability ..."

... neither do I! Just what I've read in obscure sections of some manufacturer's literature.

Re. the security side - as far as I can make out, the series connected contacts are part of the same 4-pole relay.

In some cases (and it does depend on the manufacturer and device) series-connected relay or contactor contacts allow the device to break more inductive DC current (especially at higher DC voltages). Series-connecting the contacts gives a greater arc path when the contacts open, however in this case the load appears not to be inductive DC. Perhaps someone misread the data sheets and thought that it would increase the AC current rating instead, it doesn't look like it is doing anything constructive.

That's the one - the blurb was about DC ratings - and in one section, it talked about the contact lifetime (in terms of number of operations), and in another about the breaking capacity. As I recall (through fog) there seemed to be some kind of inverse relationship, so selecting a suitable device was critically dependent on the duty cycle.

In this application, I can't see any reason for the wiring used. I can only surmise that 4-pole contactors were on hand, or easier to obtain, and "how can we explain away the redundant contacts if they're not connected to something/anything on the schematic?"

"how can we explain away the redundant contacts if they're not connected to something/anything on the schematic?"

How about expansion or backup?

But seriously, in general for standard small contactors I have found that once you get to a certain size contactor (approximately 11kW) the number of poles change from 4 (three phases + spare auxiliary rated at same current) down to 3 (three phases with auxiliary contacts as an add-on option). Makes sense as the auxiliary contact is generally only going to be used for low current control purposes.

Doesn't make sense to buy a really big contactor with three poles instead of using a correctly rated contactor with 4 poles (one of which isn't used).

Never underestimate the ability of even qualified electricians to adopt useless techniques. Also consider the cock up rather than conspiracy theory.

No body ever went bust over estimating the illogical nature of man

There could be reason – don't just think it is for no reason.

It is a 4 pole relay and it is tested (certified) with having current in each of the poles. The thermal over current is operated by the heating effect of current passes through all of the 4 poles. If less than 4 poles is used, the relay will not trip even if the over current passes through the relay because the heating effect of the current passing through 3 or 2 poles is not enough for the trip. So leave the wiring as is for the correct protection of the heater.

- MS

Hang on, I have never come across a contactor thermal overload that requires the use of all 4 poles (see my previous comments on the difference between a standard three pole and 4 pole contactor). Even the thermal overloads for the 4 pole contactors only had three terminals. Additionally the contactor thermal overload monitors each phase to trip the contactor in the event of a phase to phase or phase to ground overcurrent (not just a three phase overcurrent situation).

Are you talking about some special application thermal overload I am not aware of?

In the case of a single-phase motor being wired through a three-pole overload relay, one of the wires needs to be looped through two sets of contacts, so that all three contacts are used. This is to provide the proper amount of heat for the overload relay to sense it. Msamad describes the basic idea well, but this is the first time I have seen it applied to a heater rather than a motor. The heater relay is just a contactor, not an overload relay. In this example, the extra loop seems unusefully redundant. Can you ask the manufacturer what was in mind?

I am not sure why they only did it to one leg but one reason for looping it through 2 contacts is to reduce arc flash on the contact. We used to do this regularly when switching higher voltage DC (230VDC) on the overhead cranes. It was explained to me that; because the contacts do not make at exactly the same time or bounce exactly the same the arc is reduced by the mismatch. I would assume that, regardless of the mismatched timing, by doubling the air gap as the two contacts approach would reduce the ability to create an arc at that voltage by half.

Thanks,

Rick

This is the situation. The only available power supply in such space area of installation is 230VDC which is intermittently turned on and off [75% continuously cycling ...] On this space area is a need to power a device drawing a 5A load at 10VDC. Actually I had one...and another one...but they all burnt because they are not designed for the intermittent factor of being turned on and off...24 hours all week...

any advice is a relief...I need help on drawing applicable circuit to utilize the existing

230VDC supply into 10VDC sufficient enough to feed 5A DC load cycling 75% continuously turn on and off without burning.

I have been following this thread with interest and I suppose I should add my two-cents.

It would appear that the extra set of contacts that are wired in series with the third load was a mistake. Wiring two sets of contacts in series increases the arc-quenching capability and a set in parallel would increase the current carrying capacity. If there is any other reason for this, it eludes me. In any case, it does not harm anything to keep this, as-designed configuration. Maybe the designer simply wanted to utilize all the terminals on the contractor and not leave any unused?

I have been following this thread with interest and I suppose I should add my two-cents.

It would appear that the extra set of contacts that are wired in series with the third load was a mistake. Wiring two sets of contacts in series increases the arc-quenching capability and a set in parallel would increase the current carrying capacity. If there is any other reason for this, it eludes me. In any case, it does not harm anything to keep this, as-designed configuration. Maybe the designer simply wanted to utilize all the terminals on the contractor and not leave any unused?

We will most likely leave these relays as is for now. The one reason I have asked, is because the relays are getting very old (probably obsolete) and we may need to update some of them soon, I was Planning on replacing them with three pole relays and leave out the loop, since some of the loads do not have the loop anyway and each feeds the same type heating element. I figured that would work for us. Thank you all for your input it has been very helpful.

R W

Because your three loads are primarily resistive in nature, there is no need for any arc-quenching

Correct, the question specifically mentions resistive heating elements. Series connected contacts for arc quenching inductive and/or DC loads (while a valid method for these types of load) doesn't come in to it in this example.

This is done in case one set of contacts weld together or become damaged while the rest of the contacts still work. Some companies do this as a standard procedure to ensure if one set of contacts is damaged power can still be shut off. This may not make sense to engineers however maintenance people appreciate it.

If that were the case then , the designer would have used two-sets of contacts for each of the three equal loads, wouldn't he?

It depends on the load. If they are all equal individual loads you would be correct. The question was why this was done. My answer was based upon what I have observed in the field and talking to the designers and electricians who put these circuits in. I did not say it was correct just one reason why it may have been done this way. I did not see any other practical explanations.

Always good information, thanks to all