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overload relay, thermal overload relay, overcurrent relay (difference)

English translation: May be the same, may be different

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14:48 Jun 23, 2004
English to English translations [PRO]
Tech/Engineering - Electronics / Elect Eng
English term or phrase: overload relay, thermal overload relay, overcurrent relay (difference)
I need to check the translation of these terms in my target language, as it is rather strange and I think it needs to be corrected

do you happen to know if:
- is an overload relay always the same as "thermal overload relay"?
- what is exactly the type of load that is "over", in excess? amps?

thanks!
Elena
Elena Ghetti
Italy
Local time: 19:19
English translation:May be the same, may be different
Explanation:
Reading:

electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Electrical_Distribution_Protection_Equipment/Circuit_Breakers

and:

Overcurrent Relays
Electromagnetic attraction, induction, and solid-state overcurrent relays are usually used for short-circuit protection of low-voltage systems. When a relay operates, it initiates a circuit breaker tripping operation to isolate the fault from the rest of the system. Overcurrent relays with voltage control can distinguish between overload and fault conditions because a short circuit on an electrical system is always followed by a corresponding voltage dip: whereas, an overload causes only a moderate dip.

There are two types of overcurrent relays:
Instantaneous overcurrent relays operate without intentional time delay.
Time delay relays to allow transient currents caused by sudden overloads of brief duration. Most overcurrent relays are of this type, with time delay features to allow current several times in excess of the setting for a limited time without closing the contacts. A relay with an inverse time characteristic operates faster as current increases.
www.usbr.gov/power/data/fist/fist3_9/3_9_3.htm

my conclusions are:

overload relay – may include rapid response to overload

thermal overload relay – (always) slower response to overload

If it is an electric motor, a higher (mechanical) load will give rise to a higher current – hence overload = overcurrent. Overload will also equate to overcurrent if the supply voltage is essentially constant (whatever the equipment).

Overload may also be applied to the electrical power source, which may well need protection from such events.

None of this prevents 'overload relay', 'thermal overload relay', and 'overcurrent relay' referring to the same item in your text.

--------------------------------------------------
Note added at 2004-06-24 10:49:50 (GMT)
--------------------------------------------------

As you may know it is usually possible to convert one energy form to another:

heat > mechanical energy > electrical energy in a turbo-generator
gravitational potential energy > mechanical energy > electrical energy in a hydro-electric power station
heat > electrical energy in photo-voltaic cells
electricity > heat in household heating appliances
electricity > mechanical energy in an electric motor

Power, whether it is heat, mechanical or electrical can always be given in units of watts. Electrical power is the product of voltage and current, i.e. -

Power (watts) = Current (amps) x Voltage (volts)

In the case of supply from the mains (i.e. ultimately the national grid system) the voltage will essentially remain constant whatever the load – the house wiring would melt,for example, if the fuses/circuit breakers didn\'t intervene, but the supply voltage would not drop.

Thus if some windings of an electrical element in a room heater were to short together, without a protection device, the current would increase because its resistance is lower and hence its power consumption. It could be said to be in overload in respect to its rated capacity.

An electric motor is a little more complicated. Here there is the mechanical load (the useful motion the motor is producing) and the electrical load the electrical power the motor is consuming. In general (and particularly with series wound electric motors), “the physics” of the set up will mean that the higher the mechanical load (the greater the resistance to motion the motor has to work against) the higher the current it will draw. Thus there may be a mechanical overload in turn giving rise to an electrical overload or there may just be an electrical overload (caused by shorting of the turns/wires on the armature, for example).
Selected response from:

Robert Tucker
United Kingdom
Local time: 18:19
Grading comment
thanks a lot everybody for your clear explanations. I changed the translated text choosing a different translation for each term
4 KudoZ points were awarded for this answer

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Summary of answers provided
4 +3May be the same, may be different
Robert Tucker
5They are all different (Why else will they give different names?)Ramesh Madhavan
4See explanation below...
Tony M


Discussion entries: 1





  

Answers


1 hr   confidence: Answerer confidence 4/5Answerer confidence 4/5 peer agreement (net): +3
May be the same, may be different


Explanation:
Reading:

electronic-components.globalspec.com/LearnMore/Electrical_Electronic_Components/Electrical_Distribution_Protection_Equipment/Circuit_Breakers

and:

Overcurrent Relays
Electromagnetic attraction, induction, and solid-state overcurrent relays are usually used for short-circuit protection of low-voltage systems. When a relay operates, it initiates a circuit breaker tripping operation to isolate the fault from the rest of the system. Overcurrent relays with voltage control can distinguish between overload and fault conditions because a short circuit on an electrical system is always followed by a corresponding voltage dip: whereas, an overload causes only a moderate dip.

There are two types of overcurrent relays:
Instantaneous overcurrent relays operate without intentional time delay.
Time delay relays to allow transient currents caused by sudden overloads of brief duration. Most overcurrent relays are of this type, with time delay features to allow current several times in excess of the setting for a limited time without closing the contacts. A relay with an inverse time characteristic operates faster as current increases.
www.usbr.gov/power/data/fist/fist3_9/3_9_3.htm

my conclusions are:

overload relay – may include rapid response to overload

thermal overload relay – (always) slower response to overload

If it is an electric motor, a higher (mechanical) load will give rise to a higher current – hence overload = overcurrent. Overload will also equate to overcurrent if the supply voltage is essentially constant (whatever the equipment).

Overload may also be applied to the electrical power source, which may well need protection from such events.

None of this prevents 'overload relay', 'thermal overload relay', and 'overcurrent relay' referring to the same item in your text.

--------------------------------------------------
Note added at 2004-06-24 10:49:50 (GMT)
--------------------------------------------------

As you may know it is usually possible to convert one energy form to another:

heat > mechanical energy > electrical energy in a turbo-generator
gravitational potential energy > mechanical energy > electrical energy in a hydro-electric power station
heat > electrical energy in photo-voltaic cells
electricity > heat in household heating appliances
electricity > mechanical energy in an electric motor

Power, whether it is heat, mechanical or electrical can always be given in units of watts. Electrical power is the product of voltage and current, i.e. -

Power (watts) = Current (amps) x Voltage (volts)

In the case of supply from the mains (i.e. ultimately the national grid system) the voltage will essentially remain constant whatever the load – the house wiring would melt,for example, if the fuses/circuit breakers didn\'t intervene, but the supply voltage would not drop.

Thus if some windings of an electrical element in a room heater were to short together, without a protection device, the current would increase because its resistance is lower and hence its power consumption. It could be said to be in overload in respect to its rated capacity.

An electric motor is a little more complicated. Here there is the mechanical load (the useful motion the motor is producing) and the electrical load the electrical power the motor is consuming. In general (and particularly with series wound electric motors), “the physics” of the set up will mean that the higher the mechanical load (the greater the resistance to motion the motor has to work against) the higher the current it will draw. Thus there may be a mechanical overload in turn giving rise to an electrical overload or there may just be an electrical overload (caused by shorting of the turns/wires on the armature, for example).

Robert Tucker
United Kingdom
Local time: 18:19
Native speaker of: Native in EnglishEnglish
PRO pts in category: 12
Grading comment
thanks a lot everybody for your clear explanations. I changed the translated text choosing a different translation for each term

Peer comments on this answer (and responses from the answerer)
agree  Pike
32 mins

agree  Rahi Moosavi
44 mins

agree  ZAMOLXIS
1 hr
Login to enter a peer comment (or grade)

16 hrs   confidence: Answerer confidence 4/5Answerer confidence 4/5
See explanation below...


Explanation:
An 'overload relay' is the more general and non-specific term.

A 'thermal overload relay' could mean one of 2 things:

an overload relay that detects the overload by thermal means [usual meaning]
but in evevryday (rather sloppy, ambiguous use), it is just possible that it also can mean a relay that protetces against thermal overload in other words, a relay that specifically detects overheating of a component --- but this is the lleast likely meaning

an 'overcurrent relay' is simply a relay designed to protect against excess current --- usually, this amounts to the same as an 'overload relay', but in certain contexts there could be a nuance in meaning between them.

Please note that most protection devices would more usually be referred to as 'circuit breakers'; 'relay' would mean a separate device that has 2 parts: some means of detecting the fault condition and turning the relay on/off as appropriate; and the actualy relay itself, just an electrically operated a switch, if you like. This latter, more complicated version, is perhaps less common, but may be used where complex, inter-related protection is required, or where the fault conditions being protected against are indirect.

Note that in some languages, 'relay' appears as a miss-translation for 'contactor' (a special kind of electro-magnetically-operated s switch for high power use)

And yes, Elena, 'load' usuaully DOES imply current, though of course, that's not truly its real, underlying meaning. If you like, the electrical supply provides a voltage (normally pretty unvarying), and the 'load' (motor, light-bulb, etc.) 'draws' current from it (in fact of course, it is the voltage that forces the current through the resistance of the load). So in the event of a fault in the equipment, one of the most likely failure mechanisms is something that will cause excess load current [= current in passing through the load]--- and this is what we refer to as 'overload' --- the word 'current' being usuaully taken as read. Of course, other kinds of 'overload' are possible (for example, mechanical: trying to lift too heavy a weight, etc.), but in an electrical context, 'current' is the most likely contender.

Note that many active over-current protection devices [= circuit breakers, as distinct from fuses, which are 'passive' devices] use either thermal, or magentic, or a combination of the two, means to detect the overcurrent.

In 'thermal', the 'more-than-normal' current produces a heating effect on (for example) a bimetallic strip, which then opens the 'switch' part of the breaker; this can have a slower reaction time (as it heats up), and the reaction time may be dependent on how severe the overload is. But it has the advantage of ignoring tiny, harmless, momentary overloads (surges). The magnetic kind uses (a sample of) the current being drawn by the load passing through a coil to 'trip' the switch in the event of excess current. Very good, but can be oversensitive to minor overloads.
Hence the combined version, where the thermal part will detect a long-term slight overload, and the magnetic part can be designed to quickly detect a major overload --- and so on.

HTH!

Tony M
France
Local time: 19:19
Specializes in field
Native speaker of: Native in EnglishEnglish
PRO pts in category: 296
Login to enter a peer comment (or grade)

1 day52 mins   confidence: Answerer confidence 5/5
They are all different (Why else will they give different names?)


Explanation:
It is not possible to control big machines directly. You need to do it indirectly to save damage to the big machine. It is more costly to repair a damaged big machine than to repair the controlling equipment. Relays are instruments which react to a given input and "relay" the information ["OFF" or "ON" command] to the BIG Machine. The big machine will then react in a slower manner so as to prevent any damage to itself. Overloading causes damage to any machine. Overload relays are those that detect overloading and tell the big machine to standby or shut down. This can be done by many types of relays. A thermal relay relies on heat from over load to send this command (The Thermostat in your refrigirator sends an "ON" command when the temperature is high). An over current relay uses execess current to detect the over load and then send the appropriate command.

Ramesh Madhavan
Local time: 22:49
Specializes in field
Native speaker of: Native in EnglishEnglish, Native in TamilTamil
PRO pts in category: 32
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