gG fuses and motor rated

[Jako13!]

if I have a 20amp gG fuses and a 20M25 the continuous rating is the same but would the 20M25 behave more like a 25 amp fuse under earth fault and require a smaller ZS to operate in the same time as the gG fuse?

 

[pc1966]

I suspect this is one for @Julie.

My understanding is the likes of 20M25 would be 20A I/t curve at moderate overloads, and 25A at higher overloads (peak motor and fault clearing).

Certainly a check with the manufacturer's data would be in order!

 

[Julie.]

I suspect this is one for @Julie.

My understanding is the likes of 20M25 would be 20A I/t curve at moderate overloads, and 25A at higher overloads (peak motor and fault clearing).

Certainly a check with the manufacturer's data would be in order!

The letters in gG and gM type fuses relate firstly to the trip starting at rated current in the very long term (g), and the second whether it is general (G) or motor (M)

A 20m25 is a gM fuse - so for use on a motor

A gG will trip faster at higher currents, so would allow a higher Zs - looking at the example curves below a 20A gG requires 110A or so for 0.4s whilst the gM would need 150A or so.

EDIT:
For completeness, if the first letter is (a), then it will not provide overload protection only fault current protection, there are also (R) & (S) for rectifiers and semiconductors (2nd uppercase letter) and some others I can't remember!

 

[Jako13!]

Thanks Julie so there’s no problem changing a gG to a gM type as long as the ZS is compliant. My thinking is if you have a 32gG could we change this to a 32M63 for inrush as it’s continuous rating is 32 but as you say will need more current fault current to blow in the same amount of time as the gG type?

 

[ImpededLoop]

As I understand it, the cable needs to be capable of carrying the current of the lower rating and the ZS compliant with the upper rating.

Great question by the way, thank you!

 

[Julie.]

Thanks Julie so there’s no problem changing a gG to a gM type as long as the ZS is compliant. My thinking is if you have a 32gG could we change this to a 32M63 for inrush as it’s continuous rating is 32 but as you say will need more current fault current to blow in the same amount of time as the gG type?

Yes, providing
1) The disconnection time is still achieved for fault condition (Zs)
2) The overload section still provides sufficient protection

So for example say we have a 32A load which is still OK if it takes 3x this for 1 min (or else the cables will overheat or the internal transformer within the equipment etc).

A 32A gG would trip in 7 secs at 100A - so ok.

But a 32M63 would take 1000 seconds to trip - over 1/4 hour!

They are significantly different.

Similarly the 32A gG needs around 170A for 0.4s whilst the 32M63 needs something like 650A for 0.4s

Again significantly different.

(So something like 1 ohm when measured cold for a gG and 1/4 ohm for 32M63 gM)

 

[Julie.]

As I understand it, the cable needs to be capable of carrying the current of the lower rating and the ZS compliant with the upper rating.

Great question by the way, thank you!

Yes, but if you run the numbers, you will be quite surprised on just how different the two fuses are.

 

[Jako13!]

Yes, providing
1) The disconnection time is still achieved for fault condition (Zs)
2) The overload section still provides sufficient protection

So for example say we have a 32A load which is still OK if it takes 3x this for 1 min (or else the cables will overheat or the internal transformer within the equipment etc).

A 32A gG would trip in 7 secs at 100A - so ok.

But a 32M63 would take 1000 seconds to trip - over 1/4 hour!

They are significantly different.

Similarly the 32A gG needs around 170A for 0.4s whilst the 32M63 needs something like 650A for 0.4s

Again significantly different.

(So something like 1 ohm when measured cold for a gG and 1/4 ohm for 32M63 gM)

Thanks Julie So the 32M63 taking over 15 mins to trip is too long to protect the cable for overload?

I’m thinking practically if it’s fitted on a motor starter with an overload downstream would the overload not protected the cable for overload?

 

[Julie.]

Thanks Julie So the 32M63 taking over 15 mins to trip is too long to protect the cable for overload?

I’m thinking practically if it’s fitted on a motor starter with an overload downstream would the overload not protected the cable for overload?

Yes if you have other overload protection then that would be OK.

How are you for Zs though?

~0.26 ohm or less?

 

[Jako13!]

Yes if you have other overload protection then that would be OK.

How are you for Zs though?

~0.26 ohm or less?

Oh no it was just an example I’ve got a feeder at work that feeds a panel and it’s on 25amp gG fuses then in the panel we have two motors on 16amp type D breakers and it has bad discrimination as the main fuses sometimes blow if both pumps come on. Was seeing if I could get away with motor rated fuses.

So having overload protection downstream we can get away with bigger fuses up stream as long as the cables protected against earth faults/short circuit?

 

[Julie.]

Oh no it was just an example I’ve got a feeder at work that feeds a panel and it’s on 25amp gG fuses then in the panel we have two motors on 16amp type D breakers and it has bad discrimination as the main fuses sometimes blow if both pumps come on. Was seeing if I could get away with motor rated fuses.

So having overload protection downstream we can get away with bigger fuses up stream as long as the cables protected against earth faults/short circuit?

Yes if you a feeding a motor with its own overload protection; in fact you could use aM fuses if you have overload protection (at either end of the cable) not just gM.

It doesn't matter which end of the cable overload protection is fitted, but fault protection must be at the feeding end, not the load end.

BUT

Be careful, one circuit feeding a motor with overload protection is one thing, feeding a control panel, with lots of opportunities for an overload cannot rely on the protection being on certain portions of the end load/circuit.

So feed a couple of motors each with their own ol protection - fine, no problem.

Feed the same two (with their own ol protection) when controlled by an intelligent panel (say there is a control circuit transformer, or a plc, or other aspects) - what is providing the overload protection for these parts of the control panel?

If you provide ol protection for both motors AND specific ol protection for the controls, then providing these can never add up beyond the cable rating, then yes it would be ok.

 

[Jako13!]

Oh no it was just an example I’ve got a feeder at work that feeds a panel and it’s on 25amp gG fuses then in the panel we have two motors on 16amp type D breakers and it has bad discrimination as the main fuses sometimes blow if both pumps come on. Was seeing if I could get away with motor rated fuses.

So having overload protection downstream we can get away with bigger fuses up stream as long as the cables protected against earth faults/short circuit?

Yes if you a feeding a motor with its own overload protection; in fact you could use aM fuses if you have overload protection (at either end of the cable) not just gM.

It doesn't matter which end of the cable overload protection is fitted, but fault protection must be at the feeding end, not the load end.

BUT

Be careful, one circuit feeding a motor with overload protection is one thing, feeding a control panel, with lots of opportunities for an overload cannot rely on the protection being on certain portions of the end load/circuit.

So feed a couple of motors each with their own ol protection - fine, no problem.

Feed the same two (with their own ol protection) when controlled by an intelligent panel (say there is a control circuit transformer, or a plc, or other aspects) - what is providing the overload protection for these parts of the control panel?

If you provide ol protection for both motors AND specific ol protection for the controls, then providing these can never add up beyond the cable rating, then yes it would be ok.

Thinking about it I think the control circuit is protected by fuses think there’s reduced tails off the main isolator and 6amp fuses into a transformer. And in parallel are the two pumps. So for the tails for the transformer overload would be protected downstream by the 6amp fuses wouldn’t it?