rcd
I supposed there is a sense coil inside the RCCB (RCD)of this tankless water heater. But why is there a fuse inside one of the load side to the circuit? Isn't the round circle the sense coil where the 2 load wire passes? Why isn't the fuse in one of them?
Part of
What would be the safety approach for the secondary side? Can we say that the transformer already isolates the earth so an RCD is not necessary both at secondary and primary sides?
Yes, but only until the first earth fault occurs. Until that happens the circuit is isolated and theoretically touching either wire will not result in electric shock. In practice, capacitive coupling between the primary and secondary may cause a small current to flow.
The problem is that there is no monitoring of the circuit so the user will not be aware of the first fault grounding one of the output wires thereby making the second one live.
Or is there still some benefit to have an RCD before the isolation transformer.
It will protect the user if the transformer chassis were to go live.
I mean if the secondary in above diagram is earthed accidentally the isolation will be defeated;
Yes, but you won't get a big fault current as you would by accidentally shorting the L wire to earth. This can be very useful in fault finding, for example.
Clarification 1: If you plug a defective piece of equipment into the mains and there is an L-E fault a very large fault current may flow. With the equipment plugged into the secondary no fault current will flow. This gives you time to trace the fault.
... in that case would the RCD in primary side will still trip if someone touches the live chassis of the load?
No. The L + N current in the primary would still sum to zero so the RCD would not trip.
simulate this circuit – Schematic created using CircuitLab
Figure 1. Float monitoring.
By adding a couple of very low wattage (high resistance) lamps or secondary voltage-rated AC LED indicators the secondary voltages will be pulled to centre around earth voltage (0 V). For example, if the secondary voltage is 200 V then the lamps will cause the secondary to be like a split-phase 100 - 0 - 100 secondary. Both lamps will glow at half voltage.
If an earth fault occurs on one 'phase' that lamp will go out and the other will go to full brightness.
This may be useful in your application.
From the comments:
- "Yes, but only until the first earth fault occurs." Yes to what?
See Clarification 1.
- About having RCD before primary first you write "It will protect the user if the transformer chassis were to go live" then you write "No. The L + N current in the primary would still sum to zero so the RCD would not trip."
The transformer itself has a metal core and may have a metal case. The RCD would protect the user should s/he touch the live core or case.
RCDs work by monitoring the live and neutral wires by passing them both through what is a small current transformer. If everything is OK the current in on the live wire returns on the neutral and cancels out exactly resulting in a sum of zero. If there is an earth fault some of the current returns to the supply via the earth path, the neutral current is reduced and now the sum of live and neutral currents is non-zero, the RCD detects this and trips in milliseconds.
There are many different classes of RCD's and GFCI's. The 5mA 5 second test is the most common residential for bathroom grounded outlets or wherever secondary earth paths exist. ( see AC-2)
The selection criteria is based on location between source and load, country laws , and to avoid nuisance trips yet to prevent:
The typical HIPOT leakage test in America is < 100uA using HVDC to eliminate the AC filter earth current but where 500uA is allowed per unit which comes from the EMI ground noise PI filter current. Up to 2.5mA earth current is allowed per system that contains many PSU's.
Both RCD's and GFCI's measure the differences in line current to magnetically activate a relay. The construction depends on the current rating and residual difference threshold and thus the acceleration which affects the response time. Higher thresholds or time delays are selected to avoid nuisance trips such as from transient voltage suppressor currents to to earth.
Although a 5mA threshold device is more protective than 30 mA device, in residential areas with high humidity and dust, the device may false trip until the moisture or contaminants that cause the internal leakage to earth evaporate or are heated away.
Residential RCD's with 30mA threshold may be common but in America GFCI's now must have a 5mA/25ms threshold on outlets near moist connections to earth. Laundry room, unfinished concrete basement, outdoors, kitchen, bathroom. Old houses with 2 wire outlets may use these in place or rewiring the house with 3 wire if labelled as "no equip. ground".
Best Answer
The fuse is part of the RCCB control circuitry.
I suppose this is an North American system where RCCB and GFCI require selftest functionality. (UL 943)
The control circuit performing this test has a fuse to prevent fire in case of a failure.
This selftest is a regular intentional current across the sensing coil to make sure it detects.
It is wired on the load side so it will also be off in case of a fault.