Overload protection

[Chris.J]

I have a 7.5KVa single phase generator (240V, 32A).
Want to protect it from overload, would a 32A MCB or wire fuse be best?
It is often run at full rating, with occasional 5A overload.
This it can take for short periods (it's a big old beast), but I would like some way of remotley indicating that I am over the 32A.(ring a bell,or flash a strobe light etc)
( I don't want the MCB/fuse to blow at this small overload).
Decades ago, I would have used an Induction type relay, but these don't seem to be around any more, so I assume there is something modern that does the same job?
What can I use?
BTW, I monitor the current using a 60:1 CT into a 1A meter, so could use this current to trip some sort of relay when 0.5A is exceeded.
Help appreciated.

 

[ban-all-sheds]

Could you use one of these

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=26215&item=7533528385&rd=1
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=26215&item=7533528474&rd=1
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=26215&item=7533528499&rd=1
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=26215&item=7533528442&rd=1

to monitor the transformer output?

 

[Spark123]

Another option is RS part no 452-6821 which is a current operated relay.

 

[Lectrician]

Or get a generator suitable for your needs - 5amp............joke???

 

[Chris.J]

Thanks guys, am following up both leads.

 

[Chris.J]

Re the Siemens relays on eBay. The guy selling them does'nt have the data sheet. Says there are 7 front terminals (NC pair, NO pair, plus T1, T2, T3), & 3 pins out the back.
Downloaded the data sheet from Siemens last week, but it's encryped, so can't view it. And Siemens must have reorganised their site over the w/e as it's not there now.
Looks to me as though it might be for 3 phase use?
Ban-all-Sheds, have you used these, are you able to advise.
regards, Chris.J.

 

[Spark123]

I have used similar thermal overloads before. They are not designed to be used for what you require, they are designed for use in 3phase motor control circuits (to trip a contactor circuit). They are used due to the time delay (heating) for when the motor is starting. The motor will require to be running at overload current for a set time before the overload (control contacts only) trip. If you require an immediate indication of overload these will be of no use. Another problem you will find is they will warm up giving faster trips and may eventually stop working (as they are mechanical) or burn out. They are also designed to work on 3phase balanced systems hence a single phase system connected across 1 winding may not cause correct operation. If you loop through all 3 heating coils in series volt drop may become apparrent. Similarily if you connect parallel there may not be enough heat generated to trip. I think the RS relay is a better device for your application (except for the price!!).

 

[Chris.J]

Thanks Sparks123. You have more or less confirmed my suspicions that it's a 3 phase device.

The RS unit will do the exact job, so think I will have to go for that.
Regards.

 

[ban-all-sheds]

Good job I said "Could you use one of these ... to monitor the transformer output?", and that you checked....