Output Relay Modules

The design of output relay assemblies is a little more complicated. Seldom are they performed as separate modules, such as the relays manufactured by the Chinese firm Nari-Relays (see Figure 2.12).

An output relay module contains a number of relatively high-power elec-tromechanical relays designed for directly enabling a high-voltage coil deac-tivating switch or high-power intermediate relay with a mechanical lock and several low-power relays whose contacts are designed for activating the external alarm devices and circuits.

During the study of the type RCS-9681 (Nari-Relays) relay, I found a rather strange engineering solution. This solution, aimed at enhancement of the running speed, was realized with two electromechanical relays: one a rather powerful unit (ST type) with an operating time of 10–12 ms, and the other

FIGURE 2.11

Analog input module with encapsulated current transformers on a toroidal core.

a small high-speed relay (DS–P type) with a response time of 3–4 ms; see Figure 2.13.

The contacts of both relays were connected in parallel in order to combine, according to the developer, the high speed of the low-power relay with a sufficiently large switching capacity of the conventional unit. In practice, the small high-speed relay (which has small parts and small gaps between the contacts) is not suitable at all for switching loads under 220 V DC (its maxi-mum switching voltage is 125 V).

In an interview with the developers, it was revealed that they also did not take into account the fact that the process of closure is always accompanied by a contact bouncing (i.e., multiple interruptions of switched circuit after the first closing of contacts). Thus, the contacts of the miniature relay will be overloaded at the moment of closure and can simply burn out before shunt-ing with more high-capacity contacts.

It is important to note that the double-break contact systems are widely used in engineering. However, in such systems a contact with a larger gap is closed first as it has higher resistance to electric arcing, and then it is shunted with the standard silver contact (see Figure 2.14).

Since the use of miniature electromechanical relays with settings inappro-priate for DPR operating conditions has become rather common, some companies have been trying to find a way out by connecting varistors in parallel to contacts in order to facilitate the switching of the inductive load (see Figure 2.15). In this case, readers should refer to DPR types SEL-787, SEL-751, and some others, including the JS series, as miniature relays with a

Signal relays

Optocouplers

Trip relays

FIGURE 2.12

Individual output relay module manufactured by Nari-Relays.

maximum switching DC voltage of 150 V; and varistors of the type 14D431K with a clamping voltage of 710 V. It should be noted that this solution is not very effective as the overvoltage above 700 V resulting from switching the DC load contains a significant inductive component.

At low induction coefficients, when overvoltage at the contacts does not exceed 700 V, the varistor just does not work and voltage is sufficient to main-tain DC electric arcing on the contacts of the relay. In addition, it is impos-sible to perform standard conformity tests of the relay contacts shunted with the varistors (e.g., insulation resistance, and withstanding voltage).

Small relays

Optocouplers MOC8030 Power relays

Relay drivers ULN3003AP

FIGURE 2.13

Module of DPR output relays type RCS-9681 (Nari-Relays) with two relays connected in par-allel (high power and low power) in each channel. 1: Low-power output relay type DS-P;

2: high-power output relay type ST; 3: photocoupler type MOC8030; and 4: control drivers of the output relays type ULN3003AP.

2 1

FIGURE 2.14

Two-stage electromagnetic relay contact system. 1: auxiliary switching tungsten contact; and 2: the main silver contact.

Type T60 DPRs (General Electric; see Figure 2.16) are equipped with both conventional electromechanical and solid-state output relays. According to the product description (T60 Revision, 5.6x), the semiconductor output relays are equipped with special circuits “monitoring the DC voltage at open con-tacts and DC current at closed concon-tacts.” This is stated as if everything is clear and obvious … but further text brought me to a standstill: “The voltage is registered as a logical unit when the contact circuit current exceeds 1–2.5 mA and the current is considered as a logical unit when it exceeds 80–100 mA.”

This is an extremely strange explanation, to put it mildly. It is not only about the text, but also about the engineering solution. First, it provides monitor-ing of direct current only, which limits the scope of use. Second, the load current may be very small (1–3 mA) such as, for example, the current at the logical input of another DPR or sensitive electromechanical slave relay. How will the current monitoring system work in this situation? It turned out that the developers of this system took this possibility into account and offered customers the option to connect an additional external resistor in parallel to the contacts. If the voltage is 48 V, they recommend a 500 Ohm 10 W resistor.

Varistors

Two contact relays Single contact relays

FIGURE 2.15

Detail of the output relay module showing contacts shunted by varistors; DPR types SEL-787 and SEL-751.

This is a very large resistor! Can you imagine the size of the resistor you would have to use for 220–250 V? And where it would have to be installed?

The developers of the T60 have kept silent about this.

Another “invention” was the automatic cleaning of the contacts (auto-bur-nishing) of external relays that transmit signals to the logic inputs of T60.

The designers took care of the fact that if input current at the logic inputs is very low (less than 3 mA) and the external relay contacts are oxidized, the signal “cannot pass” through them.

For such self-cleaning purposes, the inputs of T60 are equipped with spe-cial nonlinear elements (obviously, they are similar to posistors) with low resistance in the switched-off (cold) mode and rapidly increasing resistance under voltage (the temperature is also increasing).

As a result, at the first moment after closure of external relay contacts, the current of 50–70 mA passes through them, and then (within 25–50 ms) it rapidly drops to 3 mA. It sounds like a nice idea if you have little knowledge of the processes at the contacts. “Obstruction” of contacts due to the oxi-dation occurs in low amperage circuits with switching voltages lower than 20–30 V. At higher voltages, black and unsightly contacts, resulting from the breakdown of very thin oxide layers, still conduct very well even the small-est currents (this is known as the freaking effect). Therefore, in terms of the actual DPR operating voltages, the problem is completely contrived, and its technical implementation is quite senseless.

Different types of modern DPRs manufactured by AREVA are equipped with electromechanical type G6RN-1 relays (see Figure 2.17) as a standard and/or a solid-state relay. Either of the relay types can be ordered. Areva

Output relays capsule

FIGURE 2.16

DPR type T60 output relay module with cover removed; one may ask, “Why are the output relays put in the casing?”

stated that the standard relay is able to switch a 250 A load within 30 ms or a 30 A load within 3 seconds at 300 V. We suggest that the reader assess the stated switching capacity of Areva DPR using the diagram taken from the relay G6RN-1 technical documentation (see Figure 2.18).

According to Areva, the solid-state relay withstands steady loads of up to 10 A. It is not really clear how small semiconductor elements without heat-sinks are capable of conducting a long-term 10 A current if it is known that small semiconductor devices without heatsinks are usually heated to a very high temperature with subsequent breakdowns under long-term currents exceeding 2–3 A.

Another solution is to use hybrid output relays, arranged in parallel con-nections to the electromechanical relay contact and semiconductor switch-ing element (see Figure 2.19). IGBT transistors with appropriate opto-isolated drivers protected from overvoltage by varistors are used as switching ele-ments. Usually, transistors have a large current margin (40–90 A) for pro-viding the necessary resistance against pulsed currents and for improving

Electromechanical relays

Relay 1 Relay 2

Relay 3

Relay 4 B14B13 B12B11 B10 B9 B8

B7 Out 1

Out 2

Out 3 Out 4

Solid-state relays

Solid-state relays +

+

+ + –

–

–

–

FIGURE 2.17

Output relay module with conventional electromechanical relays, and indication of the high-speed solid-state relays in Areva technical documentation.

reliability. In the hybrid scheme, transistors stay under current only until the contacts of electromechanical relays are closed (10–15 ms), and therefore they do not warm up even if heatsinks are missing. This idea is used in a DPR manufactured by SEL. Protection of the IGBT transistor is provided by shunting the reverse-switched diode. In the event of wrong polarity of the external load (e.g., the switch trip coil), it is activated immediately upon sup-ply of an external voltage, which can lead to serious problems.

The manufacturer of the same DPR type, SEL-487, uses a sophisticated hybrid relay supplemented by a diode bridge (type KBU4M, 1000 V, 4 A; see Figure 2.20). In this case, the IGBT transistor (type IRG4PF50, 900 V, 50 A) connected to the bridge diagonal provides switching both AC and DC load current. Diode bridges of this type have more than twentyfold short-time overload capacity (before relay contacts are closed), which makes them suit-able for switching the significant current loads.

In addition to the output relay, the module also contains a set of optocou-plers acting as a buffer between the control microchips and output relays, as well as resistors that preset the photocouplers’ operating mode.

SEL has declared that this design of the output relay makes it extremely fast acting (with a response time of 10 microseconds). The question is, who needs it if the time spent by the DPR for processing an input signal and

DC resistive load AC resistive load 100

50

10 5

1

0.11 5 10 30 50 100 300 500 1000

Switching Voltage (V) Relay G6RN-1 Maximum Switching Capacity

Switching Current (A)

FIGURE 2.18

Maximum switching capacity of miniature relay type G6RN-1 installed in AREVA DPR.

issuing an internal instruction to the output relay comes up to 20–40 ms? In addition, such a high speed may result in major troubles caused by malfunc-tions due to the short impulse and high-frequency noise.